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American history is checkered with ugly bouts of religious persecution—from Protestant mobs burning convents in the 1830s, to Henry Ford publishing anti-Semitic propaganda in the 1920s, to anti-Muslim violence after September 11. But there was one thing that, until 2016, had never happened before in the history of our country. No one had ever won the presidency on a campaign that prominently and persistently attacked a religious minority.

As a candidate, Donald Trump didn’t just demonize Muslims rhetorically. He offered specific policies that ran against our shared consensus about religious freedom. He proposed banning Muslims from immigrating to the country, claiming that Muslim refugees were “trying to take over our children and convince them how wonderful ISIS is and how wonderful Islam is.” Just as stunning, Trump said he would “absolutely” require American Muslims to register in a special database to make it easier for the government to track them. Finally, he said that “there’s absolutely no choice” but to close down some American mosques as a way of combating extremism.

Anti-Muslim animus grew as the 2016 election approached and Republican voters learned to take their cues from Trump. The percentage of Republicans who believed that at least half of Muslims living in the United States were anti-American jumped from 47 percent in 2002 to 63 percent in 2016, according to the Pew Research Center. Most shockingly, according to a Public Policy Polling survey, only half of Republicans were willing to say that Islam should be legal in America. So when Trump, a week after his inauguration, signed an executive order banning foreign nationals from seven predominantly Muslim countries from entering the United States, he was doing so with the overwhelming support of the voters who put him in office.

But then something encouraging happened. Thousands of people of different faiths flooded airports to protest the Muslim ban. The courts blocked the ban from taking effect, leading Trump to introduce what he called a “watered-down” version. Federal courts then blocked that one, too, because it still prioritized Christian refugees over Muslims. In June 2018 the Supreme Court’s five conservatives upheld a third version of the ban, revised to drop the preference for Christians and add two token non-Muslim countries (including North Korea, which of course has never been a significant source of U.S. immigration).

The Muslim ban exemplified two facts about religious freedom in America: It is deeply baked into our system, cherished as one of our most sacred liberties. Yet it is also fragile. The consensus can unravel quickly.

Donald Trump’s Muslim ban exemplified two facts about religious freedom in America: It is deeply baked into our system, cherished as one of our most sacred liberties. Yet it is also fragile. The consensus can unravel quickly.

On some level, liberals understand that this is a problem. But religious freedom is rarely top of mind on the left. To some degree, this reflects the right’s success at casting religious freedom as a conservative issue—one that typically concerns expanding the role of conservative Christianity in the public sphere. It is also because the Democratic coalition includes more atheists and nonreligious people. But it’s a mistake for liberals to ignore religious freedom. First, remember that the most successful progressive movements in history were driven in great measure by religion. Abolitionism and the twentieth-century civil rights movements were to a great degree religious crusades that drew power from their ability to use language and ideas that spoke to the fundamental beliefs of a broad range of Americans.

More important, when religious freedom collapses, it is the marginalized who suffer most. The moral commitments of liberalism thus require that the right to worship freely be defended. But in order to do that, we first need to understand the specifically American approach to religious freedom in America—an approach unique in the history of the world.

Societies have puzzled for millennia over how to have both religion and freedom. Today, most nations still have not found the right balance. More than three-quarters of the world’s population lives in countries with limited religious freedom, according to Pew, and 42 percent of nations still have an official or preferred religion. Varieties of oppression have flowered: Eastern Orthodox Christians harass Protestants in Russia, Muslims persecute Coptic Christians in Egypt, Buddhists attack Muslims in Myanmar. Even Western democracies have stumbled, as when, in 2016, French policemen forced female Muslim beachgoers to strip off their head scarves and burkinis because their religious attire showed disrespect to secularism.

By comparison, the United States was, at least until the Trump presidency, managing its religious diversity well. America is home to 350,000 houses of worship, from Adventist to Zoroastrian, from urban storefronts to Christian mega-churches that hold 40,000 people. Nearly three-quarters of Americans say they pray at least once a week. Notably, affluence has not dampened our religiosity as it has in other countries. The Pew Research Center recently mapped the relationship between wealth and religious practice. On the upper left of the chart is a cluster of countries that are religious and poor—Afghanistan, Nigeria, Guatemala. On the lower right are wealthy, secular nations, including Norway, Switzerland, and Germany. Way off by itself on the right edge of the chart is a single stray dot: the United States, wealthy and religious. America has reduced religious persecution without subduing religious passion.

But the struggle to make religious freedom real in America has been long and tempestuous. As with civil rights, the journey began with a set of ideas. The most significant visionary—and the most effective activist for religious liberty—was James Madison, who wrote the seminal treatise “Memorial and Remonstrance Against Religious Assessments,” engineered the passage of the Virginia Statute for Religious Freedom, and guided the creation of both the United States Constitution and the First Amendment. More than anyone else, Madison devised the ingenious, counterintuitive, and often misunderstood blueprint for the religious liberty we enjoy today.

Madison’s views were shaped by a shocking wave of religious persecution against Baptists near his home when he was a young man in Virginia. In 1771, in Caroline County, an Anglican minister approached the pulpit where Reverend John Waller was preaching and jammed the butt of a whip into his mouth. Waller was dragged outside and brutally beaten by a local sheriff. He then spent 113 days in jail. This was only one of 150 major attacks between 1760 and 1778 against Virginia’s Baptists, who today would be called evangelical Christians. In 1774, in a letter to a friend, Madison, then twenty-two, complained that the arrest of Baptist preachers “vexes me the most of any thing.”

Madison went on to devise a two-part formula for religious freedom. First, he argued that the best way to promote religion was to leave it alone. This was revolutionary. In all of previous human history, those who wanted to encourage religion had enlisted the government’s help. Madison believed that the state should neither constrain nor coddle religion and, above all, that it should not favor one faith over another. Even well-intentioned efforts would backfire, he insisted, sapping religion of its vitality.

Second, he wanted religion to have its own checks and balances. Skeptical of the efficacy of mere “parchment barriers”—lofty declarations of rights in constitutions—Madison believed that the surest path to religious liberty would come from a “multiplicity of sects” all jostling for followers. In a free marketplace of faiths, no one religion could dominate. Spiritual innovation would spread. New styles, denominations, and religions would continually emerge, creating still larger constituencies for religious freedom. Madison approached religion the way an early-twentieth-century progressive approached capitalism: he wanted open competition, but with rules to keep the big players from undermining the upstarts.

In the first quarter of the nineteenth century, the ratification of the First Amendment helped trigger a virtuous circle of liberalization. One by one, states dropped their religious regulations, getting rid of taxation-based establishments and most religious tests. Meanwhile, religious fervor erupted in the form of the evangelical Second Great Awakening, which fueled new denominations and styles (especially among Methodists and Baptists). Whole new religions, like Mormonism, sprouted. The two trends reinforced each other. Less regulation meant more religious newcomers, who then demanded still more freedom. Around the same time, immigration, especially a flood of Catholics from Ireland, further contributed to diversity.

In 1819, Madison concluded that the First Amendment had worked well—not because of the decline in religious persecution but because of the rise in enthusiasm: “On a general comparison of the present & former times, the balance is certainly & vastly on the side of the present, as to the number of religious teachers, the zeal which actuates them, the purity of their lives, and the attendance of the people on their instructions. . . . The number, the industry, and the morality of the priesthood & the devotion of the people have been manifestly increased by the total separation of the Church from the State.”

Religious freedom was nonetheless still in an early stage of development. Persecution of unpopular religious minorities continued into the nineteenth and twentieth centuries, in part because the First Amendment only applied to the national government. The persecution followed certain patterns. Minorities were often depicted as violent and too alien to ever fully blend into America. An 1838 editorial in a Missouri newspaper declared of Mormons, “Their manners, customs, religion and all, are more obnoxious to our citizens than those of the Indians, and they can never live among us in peace. The rifle will settle the quarrel.”

Harsh words like these led to horrific acts. In the fall of 1838, Missouri Governor Lilburn Boggs issued Missouri Executive Order 44, declaring that “the Mormons must be treated as enemies, and must be exterminated or driven from the State if necessary for the public peace.” Three days after the order was issued, on October 30, 1838, the biggest massacre of a religious minority in American history occurred. About 250 Missourians, including a state senator, arrived at Haun’s Mill, a small Mormon community, and opened fire. The mob murdered nineteen Mormons, including children, and wounded fifteen.

Madison believed that the surest path to religious liberty would come from a “multiplicity of sects” all jostling for followers. In a free marketplace of faiths, no one religion could dominate. Spiritual innovation would spread.

Catholics were likewise thought to be unable to accept or understand American democracy. In 1835, the famous minister Lyman Beecher warned that Catholics were a “dark minded, vicious populace—a poor, uneducated reckless mass of infuriated animalism,” and that the Catholic Church was working to “throw down our free institutions.” The day after one of Beecher’s sermons, in Boston, a few thousand people gathered around the Ursuline Convent in Charlestown. (It’s not clear whether any of them had attended Beecher’s sermon.) A few hundred of the men busted through the convent gate, chanting, “Down with the pope! Down with the convent!” While the mother superior hurried the nuns and the students out the back, the men rampaged, destroying Bibles, the nuns’ belongings, and musical instruments. They raided the crypt, collecting the teeth of deceased nuns as souvenirs. Then they burned the convent to the ground while a fire company stood by and watched.

Religious minorities were often depicted as ethnically alien. In a report submitted to the U.S. Senate in 1860, military doctor Robert Bartholow described a typical Mormon: “yellow, sunken, cadaverous visage; the greenish-colored eyes; the thick, protuberant lips, the low forehead; the light, yellowish hair, and the lank, angular person, constitute an appearance so characteristic of the new race, the production of polygamy, as to distinguish them at a glance.” In 1870, Thomas Nast, the nation’s most famous political cartoonist, launched a series in Harper’s Weekly that depicted Irish Catholics as apelike trolls.

Catholics were said to be loyal to a foreign power, the pope. Nativists warned that foreign governments were not sending their best people (so to speak). Lyman Beecher warned that foreign governments were “emptying out upon our shores” so many paupers—“the sweepings of the streets”—that the result would be “multiplying tumults and violence, filling our prisons, and crowding our poor-houses, and quadrupling our taxation.”

The conflict was ugly, but, stepping back, we can see how it pushed freedom forward. Consider the bitter fight over teaching the Bible in public schools. In the early part of the nineteenth century, Protestants insisted that schools teach their translation of the Bible and the Ten Commandments. Catholics resisted. Things got ugly. In 1834, Catholic churches and houses in Philadelphia were burned to the ground and about thirty people died. But over time, Catholics made headway, through both the state courts (which increasingly recognized that requiring them to read the Protestant Bible violated religious freedom) and the ballot box (by then, Catholics made up a sizable voting bloc in many cities). By 1887, only one-third of public schools taught the Bible.

The Mormon experience showed the system at work in a different way. In 1862, in an effort to destroy the religion, the Republican Congress passed the Morrill Anti-Bigamy Act, which outlawed polygamy, annulled the incorporation of the Mormon Church, and forbade the church from owning real estate valued at more than $50,000. In 1871, Brigham Young, the head of the church, was indicted for practicing polygamy. From 1882 to 1893, nearly 1,000 Mormons were jailed. By sticking to their principles and refusing to renounce their own family structures, Mormons engaged in massive civil disobedience.

But eventually politics intervened. Mormons wanted Utah to become a state, which would give it more control over its own affairs than if it continued as a territory. Republicans had initially resisted the idea, but the calculus changed. The political balance of power was shifting from east to west as new states continued to join the union. Mormons now represented a sizable number of votes beyond Utah: from 1876 to 1879, more than 100 Mormon settlements had been established in Arizona, Nevada, Wyoming, Colorado, and other states.

Congress let it be known: We’ll allow Utah in the union—and effectively accept Mormonism as a mainstream religion—if it just renounces polygamy. So the church did something that American religions often do, but don’t like to admit: it shape-shifted to accommodate the law, agreeing to end polygamy. Congress admitted Utah into the union soon thereafter. The historian Kathleen Flake has suggested that the growing acceptance of Mormons reflected a key strain of Progressive Era politics. In the economic sphere, Progressives sought to create a set of rules that would ensure fair competition. They applied the same principles to religion. Mormonism could be tolerated as long as the LDS Church was willing to play by the same rules as other faiths did.

These accommodations hardly ended persecution. In the 1920s, a resurgent Ku Klux Klan spread anti-Catholicism around the country. In 1927, 1,000 white-robed Klansmen joined the Memorial Day parade in Queens. Police and Klan members fought, with the police claiming that the Klan had violated a pledge to go hoodless. Klan members subsequently claimed that “Native-born Protestant Americans” were being “assaulted by Roman Catholic police of New York City.” (One of the seven people arrested during the rally was Fred Trump, Donald Trump’s father. News accounts do not specify whether he was there as a Klan member or not.)

The 1928 presidential candidacy of Al Smith, the first Catholic major-party nominee, triggered allegations that a Catholic president would be beholden to the Vatican. Klansmen claimed that a photo of the recently completed Holland Tunnel in New York City actually showed a newly built secret pathway from Rome to the United States, through which the pope would arrive and take over the country. One KKK flier showed an image of a priest throwing a baby into a fire, with the title “Will It Come to This?” In Muncie, Indiana, a twofer conspiracy theory spread: the Catholics had invented a powder that would bleach the skins of black men so they could seduce and marry unsuspecting white women.

Part of the argument against Catholics was strikingly similar to modern attacks on sharia, the religious code of behavior that is part of traditional Islam. In the Atlantic, a Protestant named Charles C. Marshall cited various Vatican rulings that, he claimed, proved that Smith would have to defer to the pope and Catholic laws. To combat the claim, Smith had to vocally support the separation of church and state, putting him at odds with the Vatican. It was not the last time that American Catholic politicians would break with the pope in order to chart a course through the pluralistic U.S. system.

Smith lost in a landslide. But it turned out that 1928 saw both a massive increase in urban populations and a shift in urban voting to the Democrats, the birth of a new coalition that would sweep Herbert Hoover out of office in 1932 and lead to Catholics thereafter having tremendous political clout within the Democratic Party. “The Republican hold on the cities was broken not by Roosevelt but by Alfred E. Smith,” declared political scientist Samuel Lubell.

When religious groups are very small, however, electoral politics offers little help. The Constitution then has to assert itself—championed by an independent judiciary. The group that most tested this view was Jehovah’s Witnesses. From 1933 to 1951, there were 18,866 arrests of Witnesses for refusing to salute the flag or comply with the military draft. Mobs punished them brutally. In Litchfield, Illinois, a mob smashed Robert Fischer’s head against the hood of a flag-draped car, demanding that he salute Old Glory. In Richwood, West Virginia, Witnesses were brought to the mayor’s office, where they were roped together like cattle, at two-foot intervals, and forced to drink castor oil.

The Witnesses responded with an unprecedented wave of lawsuits that changed the course of history. At least thirty-seven religious freedom cases involving Jehovah’s Witnesses were argued in front of the U.S. Supreme Court. In Cantwell v. Connecticut, in 1940, the Court ruled for the first time that the First Amendment’s “free exercise” clause applied to state and local government, not just to Congress. In 1943, in West Virginia State Board of Education v. Barnette, the Court held that the state could not force a pair of young Witnesses to salute the flag in school. The next year, in a law review article titled “The Debt of Constitutional Law to Jehovah’s Witnesses,” retired federal judge Edward Waite asked, “If ‘the blood of the martyrs is the seed of the Church,’ what is the debt of Constitutional Law to the militant persistency—or perhaps I should say devotion—of this strange group?”

It was World War II that ultimately cemented the American idea of religious freedom. The presence of two major existential threats, fascism and communism, forced the nation to emphasize the central role that religious liberty—not just religion—played in the American identity. Franklin Roosevelt listed it as one of the Four Freedoms. Harry Truman said it was the heart of the argument against communism.

A form of competition Madison had never envisioned—competition with totalitarian foreign adversaries—was leading America’s leaders, and increasingly its citizens, to interpret the principles of the First Amendment in a new way. “Our form of government has no sense unless it is founded in a deeply felt religious faith, and I don’t care what it is,” Dwight Eisenhower famously declared. “With us of course it is the Judeo-Christian concept, but it must be a religion that all men are created equal.” Some mocked his “I don’t care what it is” line. As one critic put it, Eisenhower seemed to be “a very fervent believer in a very vague religion.” But the president had captured the way Americans were increasingly approaching faith—with a combination of passion and tolerance. Blood had been spilled. Religious freedom therefore needed to be revered and protected. It became a sacred liberty.

In the nineteenth century, anti-Catholic nativists warned that foreign governments were sending “the sweepings of the streets,” and that the result would be “multiplying tumults and violence, filling our prisons, and crowding our poor-houses, and quadrupling our taxation.”

Over the next few decades, religious freedom continued to march forward. The Supreme Court expanded the rights of religious minorities while restricting government’s role in favoring one religion over another. Political coalition building led to further thawing in the relations between Protestants and Catholics: first, on the left, as liberal Catholics and Protestants joined to support John F. Kennedy; and then in the 1970s, on the right, as conservative Catholics and Protestants joined to fight abortion and secularism.

Meanwhile, the 1965 immigration act had loosened up immigration from non-European parts of the world, altering the religious makeup of new arrivals. Up to that point, the top ten countries sending immigrants to the United States were all majority-Christian nations. But after the effects of the 1965 law fully kicked in, the melting pot became filled with very different ingredients. From 1986 to 2012, three of the top five countries sending immigrants—China, India, and Vietnam—were majority non-Christian. The Pew Research Center estimated that from 1992 to 2012, 25 percent of immigrants followed non-Christian religions, with the largest groups being Muslims (10 percent), Hindus (7 percent), and Buddhists (6 percent).

At this point, we are not only a nation of immigrants; we are a nation of religious minorities. The original American majority was composed of Anglicans and Congregationalists. Those denominations now make up 1.7 percent of the American population. Most everyone else descends from a group that was once considered a religious minority. Our system reflects that.

The terrorist attack of September 11, 2001, posed a major challenge to America’s culture of religious pluralism. President George W. Bush delayed the rise of Islamophobia somewhat with his admirable embrace of Muslim Americans. But within a year or two, religious bigotry began to show, led in part by conservative evangelicals—an irony, since evangelicals had so often been at the forefront of expanding religious freedom in America’s past. While Billy Graham had complimented Islam in 1997, his son Franklin in 2002 called Islam a “wicked, violent religion.” The popular televangelist Pat Robertson said, “Ladies and gentlemen, we have to recognize that Islam is not a religion. It is a worldwide political movement bent on domination of the world.”

Anti-Muslim sentiment grew even louder with the presidential candidacy of Barack Obama. Rumors that he was a secret Muslim, educated in a “madrassa,” spread easily from the fringes of conservative media to the minds of millions of Republican voters. During the first few years of his presidency, anti-Islamic sentiment intensified on the local level, often in the form of attempts to block the building of mosques. After someone set fire to construction equipment at a mosque site in Murfreesboro, Tennessee, one resident said, “I think it was a piece of their own medicine. They bombed our country.” Echoing the arguments once made against Mormonism, opponents argued that Islam was not a real religion and therefore not worthy of First Amendment protections.

From 1992 to 2012, 25 percent of immigrants followed non-Christian religions, with the largest groups being Muslims (10 percent), Hindus (7 percent), and Buddhists (6 percent). At this point, we are not only a nation of immigrants; we are a nation of religious minorities.

In past decades, this localized bigotry might have remained marginalized. But a media infrastructure now existed to give them national scope and legitimacy. Conservative outlets, especially Fox News, gave positive coverage to these stories and invited on “experts” to validate other lunatic ideas, such as the menace of sharia. (Sharia is similar to the Halacha rules that govern some Orthodox Jews and to Catholic canon law, which affects all practicing Catholics.) “Is Islam a destructive force?” asked Bill O’Reilly. “There are exceptions to the rule, but they are few.” Brian Kilmeade of Fox & Friends suggested, “Not all Muslims are terrorists, but all terrorists are Muslims.”

In 2011, another frequent Fox talking head picked up and ran with the anti-Muslim madness: Donald Trump. The real estate mogul began airing his anti-Muslim message during his drive to prove that Barack Obama wasn’t really a U.S. citizen. In March 2011, he said to radio host Laura Ingraham, “Now, somebody told me—and I have no idea if this is bad for him or not, but perhaps it would be—that where it says ‘religion,’ it might have ‘Muslim.’ ”

When Don from Queens became a presidential candidate, we entered uncharted waters. Religious freedom has been sustained not just by laws and court rulings but also by an informal consensus that past attacks on minority religions were fundamentally un-American. When the president of the United States doesn’t respect that idea, the consensus becomes vulnerable.

Trump’s attacks on American Muslims were indirect at first. But his rhetoric escalated heading into the first Republican primaries, a period that coincided with two terrorist attacks—the mass killing orchestrated by ISIS on November 13, 2015, in Paris and the shooting in San Bernardino, California, on December 2, when two Muslims murdered fourteen coworkers at a Christmas party. Trump deployed many of the same lines of attack used by anti-Muslim activists and previous generations of religious bigots. Echoing the old attack against Mormons, Trump insisted that Muslims couldn’t become fully American: “I’m talking about second and third generation,” he told Fox’s Sean Hannity. “For some reason, there’s no real assimilation.” And as earlier demagogues did to Catholics and Jehovah’s Witnesses, he suggested that Muslims were dangerously disloyal. “When they see trouble they have to report it,” he said. “They are not reporting it. They are absolutely not reporting it and that is a big problem.” (In fact, the evidence is overwhelming that law enforcement has been able to thwart a huge number of attacks because of the cooperation of rank-and-file Muslim Americans. According to a Duke University study, American Muslims provided tips in forty-eight of the 120 violent terrorist plots that were thwarted between 2001 and 2011.)

When Don from Queens became a presidential candidate, we entered uncharted waters. Religious freedom has been sustained not just by laws and court rulings but also by an informal consensus that past attacks on minority religions were fundamentally un-American.

Most important, in December 2015, Trump called for a “total and complete shutdown of Muslims entering the United States until our country’s representatives can figure out what is going on.” Gone was the idea that we should focus on Islamic fundamentalists or terrorists. No Muslims of any kind could be trusted. Just as stunning, Trump said he would “absolutely” require American Muslims to register in a special database to make it easier for the government to track them. And finally, he said that “there’s absolutely no choice” but to close down some American mosques.

Meanwhile, violent attacks on American Muslims multiplied. Hate crimes reported to the FBI grew 76 percent from 2014 to 2017. Almost one-third of the attacks in 2015 came in December—just one month—as Trump’s anti-Muslim campaign hit full gear. On December 10, a mosque was firebombed in Coachella Valley, California. On December 12, in Grand Rapids, Michigan, a robber called a convenience store clerk a “terrorist” before shooting him in the face. On December 24, a shooter ranted about Muslims before killing one man and injuring another outside a Muslim-owned tire shop in Pleasant Grove, Texas.

Trump continued to lead in Republican primary polls.

At the heart of James Madison’s vision was a system of fair competition among religions: the power of the state should not be used to favor one over another. Trump’s ascent to the presidency has challenged that principle directly: he proudly advertises his desire to favor one group, white evangelicals, over others, especially Muslims.

“The Christians are being treated horribly because we have nobody to represent the Christians,” Trump said during the 2016 campaign. He promised not only to protect Christians from persecution but also to restore their dominance: “We have to band together. . . . Our country has to do that around Christianity.” Although Trump has advocated a few legitimate expansions of rights for religious people generally, he mostly has defined religious liberty downward, using the concept, for instance, to justify allowing tax-exempt churches to endorse political candidates.

Meanwhile, Trump stocked his government with men allied to the most extreme anti-Muslim activists. Michael Flynn, his first national security adviser, dismissed Muslims’ claims that they should be protected by the First Amendment as a treacherous tactic. John Bolton, the current national security adviser, appointed as his chief of staff Fred Fleitz, the senior vice president of Frank Gaffney’s Center for Security Policy, one of the leading groups peddling conspiracy theories about the looming threat of sharia. After the Boston Marathon bombing in 2013, Secretary of State Mike Pompeo, then a member of Congress, claimed that the “silence of Muslim leaders has been deafening” and that therefore “these Islamic leaders across America [are] potentially complicit in these acts.”

Trump and the anti-Muslim extremists he has empowered have already degraded the basic rules that had long propelled America’s unique model of religious freedom. But things could still get much worse. After ten years of propaganda from Fox News, right-wing trolls, talk radio hosts, and now the president of the United States, a substantial minority of Americans don’t believe that Muslims are worthy of First Amendment protections. The foundation of religious freedom has been soaked with gasoline.

Now imagine there’s a large-scale terrorist attack on American soil committed by a Muslim radical. Does anyone expect Trump to caution his followers against blaming Islam as a whole? He would more likely add fuel to the fire. How many hours would pass before we heard him say, “See, I was right about the Muslims!” And since the whole thrust of the anti-Muslim movement of the last decade has been to blur the line between Muslim terrorists and ordinary Muslims, Trump’s reaction could embolden more of his supporters to take matters into their own hands. And history is full of reminders that once animus is normalized against one religious minority, others are at risk of being next in line.

It is hard to imagine mass religious violence in modern America. But remember Trump’s words when asked, in November 2015, whether he would consider shutting down mosques as president: “We’re going to have to do certain things that were frankly unthinkable a year ago.” America’s system of religious freedom has been so successful that liberals have stopped worrying very much about how to defend it. If we don’t start again, the unthinkable may become frighteningly thinkable.