Among Mormons, the proportion of the U.S. adults who claim to be Latter-day Saints was essentially unchanged, according to the study, dipping from 1.7 percent in 2007 to 1.6 percent last year.
That statistic, said Notre Dame political scientist David Campbell, is striking because it contradicts what is practically a Mormon article of faith: that The Church of Jesus Christ of Latter-day Saints, with its expanding missionary force, is the nation's fastest-growing religion.
"While many Mormons are coming in the front door," Campbell said, "many others are leaving out the back door."
Conversely, the figures counter a widespread notion among former Mormons that the LDS Church is hemorrhaging to the point its membership is shrinking dramatically.
The 0.1 percentage-point slip is seen as statistically insignificant, given the fact that fewer than 700 LDS were among the 35,071 surveyed by Pew.
The nation's adult population rose from 227 million to 245 million between 2007 and 2014. If the survey's conclusion is correct, that would mean the number of adult Mormons in America increased from 3.86 million to 3.92 million last year.
The Utah-based faith currently reports its total U.S. membership (including children) at nearly 6.5 million.
Pew's report demonstrates that America's religious makeup continues to be dynamic, with people readily switching faiths and increasingly marrying people from other traditions, says John C. Green, a political scientist who studies American religion at the University of Akron and was an adviser to the study.
"American religion is as caught up in change and innovation as any other part of American life," Green said. "That's not to say all are happy about it."
Among the survey's results:
• The decline in Christians and the rise in the unaffiliated ranks stretch across the religious landscape, affecting all regions and most demographic groups. While the change is particularly true for millennials, those born between 1981 and 1996, it holds, too, for older Americans, whites, blacks and Latinos; college graduates and high-school graduates; women as well as men.
• The West has the highest percentage of adults who are unaffiliated: 28 percent, up from 21 percent in 2007. Nineteen percent of those in the South identify as religious "nones."
• Mainline Protestant denominations continue a long slide in membership, but the world's largest Christian faith, the Catholic Church, also is losing members in a big way. (Previous studies were mixed on that.) Overall, 46.5 percent of U.S. adults now say they're Protestant (evangelicals included), down from 51.3 percent in 2007. Catholics comprised 20.8 percent of adult Americans last year, down from 23.9 percent in 2007.
• Among Protestants, evangelicals and historically black churches saw the smallest erosion in membership. Evangelicals as a share of the U.S. population fell less than 1 percentage point to 25.4 percent. The share of those affiliated with black churches fell less than half a percentage point to 6.5 percent.
• Among U.S. adults, 3.1 percent say they are atheist, up from 1.6 percent, and 4 percent say they are agnostic, up from 2.4 percent.