LOS ANGELES—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%.