In 2016, white evangelical Protestants strongly supported Donald Trump, a septuagenarian candidate who promised to make America great again, to bring back “Merry Christmas” and to protect, cherish and defend America’s Christian heritage. White evangelicals have consistently told pollsters that life in the U.S. has gotten worse since the 1950s. Nostalgia seems to be animating much of white evangelical politics.

But in longing for an American past, white evangelical Protestants may be neglecting their future. As a group, they’re drifting further away — politically and culturally — from the American mainstream. There are growing signs that white evangelical Protestantism is no longer immune to the broader social and cultural forces that are reshaping the American religious landscape.

In the aftermath of the 1960s, scholars began to note that while more liberal mainline Christian churches appeared to be shedding members, conservative and more traditionally minded churches were unaffected. In 1972, sociologist Dean Kelley published the study “Why Conservative Churches are Growing,” arguing that evangelical churches were prospering because they placed greater demands on their members. Subsequent research published decades later appeared to support this claim. Conservative churches that offered a rigorous theology were thriving, arguably because of it.

It’s a narrative that has gained widespread acceptance and has tremendous staying power. Shortly after the Broadway hit “The Book of Mormon” was released, David Brooks, for example, argued that strict observance to a consistent theology is essential for the vitality of religious communities. “The religions that grow, succor and motivate people to perform heroic acts of service are usually theologically rigorous, arduous in practice and definite in their convictions about what is True and False.”

Over the past couple of decades, though, Americans have become far more accepting on nearly every issue that fits under the rubric of sexual morality. Today, most Americans say same-sex relationships, premarital sex and having children out of wedlock are morally acceptable. And roughly three-quarters of the public has no moral qualms about divorce.

The driver behind much of this change is “generational turnover.” And so a chasm has emerged between the views of these young people and white evangelical Protestants. A PRRI survey found that 83 percent of the latter believe that sex is morally acceptable only between a man and a woman who are married, but this view is held among only 30 percent of all young adults. For many young people, white evangelical Protestants in the 21st century appear to be advocating a mid-20th century approach to sex, relationships and marriage, even as American society resembles life during this period less and less.

This may help explain why the religious profile of young adults today differs so dramatically from older Americans. Only 8 percent of young people identify as white evangelical Protestant, while 26 percent of senior citizens do.

After dominating much of American politics for the past 40 years, white evangelical Protestants are now facing a sharp decline. Nearly one-third of white Americans raised in evangelical Christian households leave their childhood faith. About 60 percent of those who leave end up joining another faith tradition, while 40 percent give up on religion altogether. The rates of disaffiliation are even higher among young adults: 39 percent of those raised evangelical Christian no longer identify as such in adulthood. And while there is always a good deal of churn in the religious marketplace — people both entering and leaving faith traditions — recent findings suggest that membership losses among white evangelical Protestants are not being offset by gains.

As a result, the white evangelical Protestant population in the U.S. has fallen over the past decade, dropping from 23 percent in 2006 to 17 percent in 2016. But equally troubling for those concerned about the vitality of evangelical Christianity, white evangelical Protestants are aging. Today, 62 percent of white evangelical Protestants are at least 50 years old. In 1987, fewer than half (46 percent) were. The median age of white evangelical Protestants today is 55.

While it is difficult to draw a direct connection between the numerical decline of white evangelical Protestants and their increasing isolation on sexual morality, the views of former evangelical Protestants provide some important clues. Analysis of a 2014 Pew study finds that former white evangelicals are far more likely than current white evangelicals to favor same-sex marriage (60 percent vs. 24 percent) and believe that society should accept homosexuality (67 percent vs. 32 percent). They are also substantially younger.

Other research also suggests that one of the prime motivators for leaving a religion is belief incompatibility. A 2016 PRRI study found that the most common reason people give up on their childhood faith is that they no longer believe in its teachings. Twenty-nine percent of Americans who have left their formative religion explicitly mention negative teachings about gay and lesbian people as a proximate cause for their disaffiliation.

Even in the face of these demographic challenges, there are few signs that evangelical Christianity will bend to the prevailing cultural winds. In 2017, a group of evangelical pastors and theologians released the “Nashville Statement” on sexual morality and gender roles, which among other things reaffirmed the group’s uncompromising opposition to homosexuality.

When World Vision, a global Christian humanitarian organization, announced in 2014 that it would hire gay Christians in same-sex marriages, it faced a huge public backlash from conservative Christians. Two days later — with complaints piling up on the charity’s Facebook page — the president reversed course and reaffirmed World Vision’s “commitment to the traditional understanding of biblical marriage.”

More recently, Rachel Held Evans, a popular evangelical author, faced substantial criticism when she publicly wrestled with the question of homosexuality and same-sex marriage. Andrew Walker, director of policy studies for the Southern Baptist Convention’s Ethics and Religious Liberty Commission, was blunt in his critique of Evans: “What she offers as an answer is unbiblical and theologically dangerous.” Evans is now attending an Episcopal church.

For young white evangelical Christians, this unyielding stance can be a source of considerable tension. If you are under the age of 30, it is increasingly difficult not to know someone who is gay or lesbian. Young white evangelicals are caught between their peers, who are predisposed to embrace cultural pluralism and express tolerance for different personal behaviors, and an evangelical tradition that staunchly resists changes in social, cultural and religious norms.

But this type of theological flexibility is what many younger evangelicals want. Nearly half (48 percent) of white evangelical Protestants under 30 say that their church should adjust traditional beliefs and practices or adopt modern beliefs and practices. Few older evangelicals agree. More than 7 in 10 (71 percent) white evangelicals over the age of 65 say their church should preserve its traditional beliefs and practices.

The relationship between evangelical Christianity and the broader culture has always been somewhat fraught. Christian Smith captured this state of affairs in his 1998 book “American Evangelicalism: Embattled and Thriving.” For many white evangelical Protestants, the feeling of being both a part of and apart from mainstream culture is entirely familiar, and perhaps inevitable. Samuel D. James, writing in the journal First Things, argued, “You cannot boil down Christianity to the parts that you are unashamed to speak about in the presence of your intelligent gay neighbor or your prayerful lesbian church member.” James’s instinct to hold the line against prevailing winds may resonate with many, but if white evangelical Protestants want to continue to be a home for younger Americans, they may have to reconsider what parts of Christianity are non-negotiable.