The religious marketplace is extremely volatile, with nearly half of American adults leaving the faith tradition of their upbringing to either switch allegiances or abandon religious affiliation altogether, a new survey finds.

The study released today by the Pew Forum on Religion and Public Life is unusual for its sheer scope, relying on interviews with more than 35,000 adults to document a diverse and dynamic American religious population.

While much of the study confirms earlier findings — mainline Protestant churches are in decline, non-denominational churches are gaining, and the ranks of the unaffiliated are growing — it also provides a deeper look behind those trends, and of smaller religious groups.

“The American religious economy is like a marketplace — very dynamic, very competitive,” the director of the Pew Forum, Luis Lugo, said. “Everyone is losing, everyone is gaining. There are net winners and losers, but no one can stand still. Those groups that are losing significant numbers have to recoup them to stay vibrant.”

The U.S. Religious Landscape Survey estimates America is 78% Christian and about to lose its status as a majority Protestant nation, at 51% and slipping.

More than one-quarter of American adults have left the faith of their childhood for another religion or no religion at all, the survey found. Factoring in moves from one stream or denomination of Protestantism to another, the number rises to 44%.

One in four adults between ages 18 and 29 claim no affiliation with a religious institution.

“In the past, certain religions had a real holding power, where people from one generation to the next would stay,” a Penn State University sociologist who consulted in the survey planning, Roger Finke, said. “Right now, there is a dropping confidence in organized religion, especially in the traditional religious forms.”

Mr. Lugo said the 44% figure is “a very conservative estimate,” and more research is planned to determine the causes.

“It does seem in keeping with the high tolerance among Americans for change,” Mr. Lugo said. “People move a lot, people change jobs a lot. It’s a very fluid society.”

The religious demographic benefiting the most from this religious churn comprises those who claim no religious affiliation. People moving into that category outnumber those moving out of it by a three-to-one margin.

The majority of the unaffiliated — 12% of the overall population — describe their religion as “nothing in particular,” and about half of those say faith is at least somewhat important to them. Atheists or agnostics account for 4% of the total population.

The Roman Catholic Church has lost more members than any faith tradition because of affiliation swapping, the survey found. While nearly one in three Americans were raised Catholic, fewer than one in four say they’re Catholic today. That means roughly 10% of all Americans are ex-Catholics.

The share of the population that identifies as Catholic, however, is fairly stable in recent decades thanks to an influx of immigrant Catholics, mostly from Latin America. Nearly half of all Catholics under 30 are Hispanic, the survey found.