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.
• Nearly 9 percent say they are nothing in particular and that religion is unimportant to them, and 6.9 percent say they are nothing in particular, but religion is important to them.
• Non-Christian religions are growing, comprising 5.9 percent of the population last year, up from 4.7 percent seven years ago. The numbers of Hindus and Muslims are mushrooming fastest, likely because of immigration from Southeast Asia, the Middle East and Africa, Green said.
• Even as their numbers decline, U.S. Christian churches are becoming more racially and ethnically diverse. The percent of nonwhites among Christians shot up by 5 percentage points, to 34 percent, last year.
"This is the twilight of the white ethnic church in the United States," said Bill D'Antonio, senior fellow at the Institute for Policy Research & Catholic Studies at The Catholic University of America. "At the same time, twilight is long."
Green, at the University of Akron, said that although the trend away from Christianity is striking, it's impossible to know whether it will be permanent. In American history, religious resurgences often have followed periods of decline.
Evangelicalism became a force in the 1980s as a response to the secularizing 1960s and '70s, which in turn were a reaction to one of the most religious times in American history: the 1950s.
"Is it bad news for organized religion? Well, yes, for some," Green said. "On the other hand, there's tremendous opportunity to convert people and recruit people if you have a religion that meets their needs."
Notre Dame's Campbell said he, too, expects churches to react. "Across traditions, we'll see folks try to win them back to the pews.
"Whether they'll succeed is an open question," Campbell added. "It would be a fool's errand to try to project too far in the future. America religion is extremely dynamic and responsive."
The Pew Research Center will continue to drill into the data gathered in its survey of more than 35,000 adults last September with an error margin nationally of less than 1 percentage point and future reports will take a closer look at particular issues.
Deeper look at the report
An analysis of Mormons, based on data from Pew's 2007 study, was published in 2009. The data so far from the 2104 report can be manipulated at an interactive website found at: http://www.pewforum.org/religious-landscape-study/