Like the tumultuous adolescent years of human development, the changes during the teen years of the 21st century disrupted American identity as we’ve known it. These transformations have come upon us quickly, upending long-standing assumptions — particularly among white Christians — about the American social fabric. And as with teenagers, they have created a lot of anxiety and fear about the future.

Of all the changes to identity and belonging, the century’s second decade has been particularly marked by a religious sea change. After more than two centuries of white Anglo-Saxon Protestant dominance, the United States has moved from being a majority-white Christian nation to one with no single racial and religious majority.

The United States has moved from being a majority-white Christian nation to one with no single racial and religious majority.

When I first identified this shift mid-decade in my 2016 book “The End of White Christian America,” I noted that the percentage of white Christians in the general population had dropped from 53 percent to 47 percent between 2010 and 2014 alone. Now, at the end of the decade, only 42 percent of Americans identify as white and Christian, representing a drop of 11 percentage points.

In the world of demographic measurement, where changes typically occur at a glacial pace, this drop in self-identified white Christians, averaging 1.1 percentage points a year, is remarkable. Changes of this magnitude are large enough to see and feel at the local level, as church rolls shrink and white Christian institutions hold less sway in public space.

In addition to white American Christianity crossing the majority-minority threshold, the last decade also saw a particularly significant decline within one subgroup: white evangelicals. While the ranks of white mainline Protestants and white Catholics have been shrinking for decades, white evangelical Protestants had seemed immune to the forces eroding membership among other white Christian groups.

But since 2010, the number of white evangelical Protestants has dropped from 21 percent of the population to 15 percent. While white evangelical Protestants have enjoyed an outsized public presence over the last four years because of their predominance in President Donald Trump’s unshakeable base, it is notable that today they are actually roughly the same size as their white mainline Protestant cousins (15 percent vs. 16 percent, respectively).

The underlying tectonic forces producing these trends are the result of both demographics and departures. According to an analysis of U.S Census population projections by William Frey at the Brookings Institution, racial changes are partly fueling this trend.

In 2017, the U.S. Census Bureau reported that, for the first time, there was an absolute decline in the country’s white, non-Hispanic population. In other words, whites not only lost ground as a proportion of the population, but in actual numbers; there were more deaths than births. The U.S. Census Bureau now predicts that the U.S. will no longer be majority-white by 2045, and among children at every age below 10, whites are already a minority.

But this is only part of the story. The simultaneous development in the religious landscape — the one that is turbocharging these trends — is the exodus of young people from white Christian churches and into the ranks of “the nones,” the growing number of Americans who claim no religious affiliation.

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As recently as the 1990s, the percentage of religiously unaffiliated Americans was in single digits. That number increased to 19 percent by 2010 and rose another 7 percentage points over the last decade to 26 percent today.

The explosive growth of religiously unaffiliated Americans is primarily driven by white Christians; African American Protestants and the relatively small block of non-Christian religious groups (e.g., Jews, Muslims, Buddhists, Hindus) are largely holding steady as a proportion of the population, while Latino, Asian American and Pacific Islander and other nonwhite Christian groups are generally growing.

The reasons for these departures from white Christian churches are complex, but many are rooted in the way that culture war politics played out over the last few decades of the 20th century. As white Christian millennials were coming of age, the rise of the Christian Right meant that the public faces of Christianity, and even religion overall, were a cadre of white male religious leaders who were almost exclusively supporting conservative Republican political candidates and who had opposing LGBTQ rights at the top of their agenda.

These commitments were an exceptionally challenging fit for a cosmopolitan cohort that leans progressive and for whom affirming LGBTQ rights is a near-consensus issue. For example, among Americans under the age of 30, only 18 percent identify as conservative while 75 percent support same-sex marriage.

Because throngs of their young people are heading for the church exits, white Christians are also graying as they are shrinking. Today, the median age of white Christians (55 years old) is 7 years older than the general population (48 years old) and 17 years older than religiously unaffiliated Americans (38 years old).

The white Christian population’s anxieties about the future as they lose traction in the present have created a nostalgia for the past that has fueled support for Trump’s “Make America Great Again” agenda, and not just among white evangelicals. Solid majorities of each white Christian subgroup voted for Trump in 2016 and, in the Public Religion Research Institute’s most recent American Values Survey, nearly 9 in 10 (88 percent) white evangelicals and approximately two-thirds of both white mainline Protestants (68 percent) and white Catholics (65 percent) oppose impeaching and removing him from office.

White Christian America’s attraction to Trump has little to do with his personality or character — a slim majority (52 percent) of white evangelicals, for example, say they wish his speech and behavior were more like previous presidents — and everything to do with something more important: their belief that “making America great again” necessarily entails restoring white Christian demographic and political dominance.

One PRRI survey question right before the 2016 election made the power of this nostalgia especially clear: “Since the 1950s, do you think American culture and way of life has changed for the better, or has it mostly changed for the worse?” Americans are divided nearly equally on this question, with 48 percent saying things have changed for the better and 51 percent for the worse. But solid majorities of white Christian groups — 57 percent of white Catholics, 59 percent of white mainline Protestants and fully 74 percent of white evangelical Protestants — believe things have changed for the worse. Among religiously unaffiliated Americans, nearly two thirds (66 percent) say things have changed for the better.

From the perspective of a healthy democratic society, one of the most alarming developments is that these trends have been compounding the political polarization in the country. Despite the demographic changes of the last decade, Republicans remain 72 percent white and Christian, three times the percentage of Democrats. And only 9 percent of Republicans are religiously unaffiliated, compared to 29 percent of Democrats.

This demographic and cultural sorting means that our partisan conflicts are increasingly driven not just by political disagreement but by entire worldviews.

This demographic and cultural sorting means that our partisan conflicts are increasingly driven not just by political disagreement but by entire worldviews that are rooted in religious, racial and generational values and identities. This arrangement leaves us ill-equipped to deal with a past that cannot be resurrected and to build a new, pluralistic future together.

Psychologists sometimes talk about the teen years as a period of “temporary insanity.” Adolescents, hurtling forward toward an unknown destination at unaccustomed speed, often assume high risks for short-term rewards and double-down on ill-conceived decisions. The teen years of the 21st century, with their massive demographic and religious changes, have produced much in our culture and politics that fit that description. Here’s hoping that the upcoming decade may find us able to accept and even embrace a future that — while different from our past — is already and inevitably well on its way.

More from our decade reflections project:

• THINKing about 2010-2019: Where we started, how we grew and where we might go

• A decade of Black Lives Matter gives us a new understanding of Black liberation

• How our phones became our whole lives in just 10 years

• College in the U.S. is at a crossroads. Will it increase social mobility or class stratification?

• The 'me too' movement's success took a decade of work, not just a hashtag. And there's more to do.

• The decade in LGBTQ: Pop culture visibility but stalled political progress

• Egg freezing and IVF in the 2010s brought us the next phase in women's lib

• How Netflix, Star Wars and Marvel redefined Hollywood — and how we experience movies

• Opioids, pot and criminal justice reform helped undermine this decade's War on Drugs

• Taylor Swift, Beyoncé, Rihanna, Gaga, Pink and Kesha cleared the way for women in the 2010s

• Climate change became a burning issue in the past decade, but also an opportunity

• Celebrities like Gwyneth Paltrow made the 2010s the decade of health and wellness misinformation