Amy Boyle grew up in the Roman Catholic Church, singing in choir and serving as an altar girl.
But Ms. Boyle, now a 35-year-old L.A. resident and mother of twin baby boys, left the faith as a young adult—joining the ranks of Americans who don’t identify with any religion, many of them former Christians. The group is growing so rapidly, it now makes up a greater share of the U.S. population than Catholics or mainline Protestants, according to a survey to be released Tuesday by the Pew Research Center on changes in religious affiliation between 2007 and 2014.
The U.S. is expected to remain majority-Christian for decades. But surveys have noted a decline in the share of Americans practicing Christianity for years, especially among Protestant faiths. Meanwhile, the number of Americans who claim no religious affiliation, known as “nones,” has climbed.
The trends “have been under way for some time,” said Greg Smith, the Pew study’s lead researcher. “But I am struck by the pace at which that group [the religiously unaffiliated] continues to grow.”
The share of Americans who say they are unaffiliated with any religion rose to 22.8% last year from 16.1% in 2007, according to the study. Over that time, mainline Protestants—including large denominations of Methodists, Presbyterians and Lutherans—dropped to 14.7% of the population from 18.1%. The share of Catholics fell to 20.8% from 23.9%.
Only evangelical Protestants—which include the Southern Baptist Convention—account for a greater share of the population than the unaffiliated, with 25.4%. The Pew study is based on interviews with more than 35,000 Americans in 2014, in all 50 states plus Washington, D.C.
The survey underscores the nation’s role in a global shift in the numbers of Christians, Muslims and those unaffiliated with any religion. That dynamic is expected to produce an equal number of Christians and Muslims in the world by 2050, according to an earlier Pew study.
The drop in the Christian share of the population has been driven mainly by declines among mainline Protestants and Catholics. Each of those large religious traditions has shrunk by approximately three percentage points since 2007. The evangelical Protestant share of the U.S. population also has dipped, but at a slower rate, falling by about one percentage point since 2007.2
Even as their numbers decline, American Christians – like the U.S. population as a whole – are becoming more racially and ethnically diverse. Non-Hispanic whites now account for smaller shares of evangelical Protestants, mainline Protestants and Catholics than they did seven years earlier, while Hispanics have grown as a share of all three religious groups. Racial and ethnic minorities now make up 41% of Catholics (up from 35% in 2007), 24% of evangelical Protestants (up from 19%) and 14% of mainline Protestants (up from 9%).