WASHINGTON — More than a quarter of adult Americans have left the faith of their childhood to join another religion or no religion, according to a new survey of religious affiliation by the Pew Forum on Religion and Public Life.

The report, titled “U.S. Religious Landscape Survey,” depicts a highly fluid and diverse national religious life. If shifts among Protestant denominations are included, then it appears that 44 percent of Americans have switched religious affiliations.

For at least a generation, scholars have noted that more Americans are moving among faiths, as denominational loyalty erodes. But the survey, based on interviews with more than 35,000 Americans, offers one of the clearest views yet of that trend, scholars said. The United States Census does not track religious affiliation.

The report shows, for example, that every religion is losing and gaining members, but that the Roman Catholic Church “has experienced the greatest net losses as a result of affiliation changes.” The survey also indicates that the group that had the greatest net gain was the unaffiliated. More than 16 percent of American adults say they are not part of any organized faith, which makes the unaffiliated the country’s fourth largest “religious group.”

Detailing the nature of religious affiliation — who has the numbers, the education, the money — signals who could hold sway over the country’s political and cultural life, said John Green, an author of the report who is a senior fellow on religion and American politics at Pew.

Michael Lindsay, assistant director of the Center on Race, Religion and Urban Life at Rice University, echoed that view. “Religion is the single most important factor that drives American belief attitudes and behaviors,” said Mr. Lindsay, who had read the Pew report. “It is a powerful indicator of where America will end up on politics, culture, family life. If you want to understand America, you have to understand religion in America.”

In the 1980s, the General Social Survey by the National Opinion Research Center indicated that from 5 percent to 8 percent of the population described itself as unaffiliated with a particular religion.

In the Pew survey 7.3 percent of the adult population said they were unaffiliated with a faith as children. That segment increases to 16.1 percent of the population in adulthood, the survey found. The unaffiliated are largely under 50 and male. “Nearly one-in-five men say they have no formal religious affiliation, compared with roughly 13 percent of women,” the survey said.

The rise of the unaffiliated does not mean that Americans are becoming less religious, however. Contrary to assumptions that most of the unaffiliated are atheists or agnostics, most described their religion “as nothing in particular.” Pew researchers said that later projects would delve more deeply into the beliefs and practices of the unaffiliated and would try to determine if they remain so as they age.

While the unaffiliated have been growing, Protestantism has been declining, the survey found. In the 1970s, Protestants accounted for about two-thirds of the population. The Pew survey found they now make up about 51 percent. Evangelical Christians account for a slim majority of Protestants, and those who leave one evangelical denomination usually move to another, rather than to mainline churches.

To Prof. Stephen Prothero, large numbers of Americans leaving organized religion and large numbers still embracing the fervor of evangelical Christianity point to the same desires.

“The trend is toward more personal religion, and evangelicals offer that,” said Mr. Prothero, chairman of the religion department at Boston University, who explained that evangelical churches tailor many of their activities for youth. “Those losing out are offering impersonal religion and those winning are offering a smaller scale: mega-churches succeed not because they are mega but because they have smaller ministries inside.”

The percentage of Catholics in the American population has held steady for decades at about 25 percent. But that masks a precipitous decline in native-born Catholics. The proportion has been bolstered by the large influx of Catholic immigrants, mostly from Latin America, the survey found.

The Catholic Church has lost more adherents than any other group: about one-third of respondents raised Catholic said they no longer identified as such. Based on the data, the survey showed, “this means that roughly 10 percent of all Americans are former Catholics.”

Immigration continues to influence American religion greatly, the survey found. The majority of immigrants are Christian, and almost half are Catholic. Muslims rival Mormons for having the largest families. And Hindus are the best-educated and among the richest religious groups, the survey found.

“I think politicians will be looking at this survey to see what groups they ought to target,” Professor Prothero said. “If the Hindu population is negligible, they won’t have to worry about it. But if it is wealthy, then they may have to pay attention.”

Experts said the wide-ranging variety of religious affiliation could set the stage for further conflicts over morality or politics, or new alliances on certain issues, as religious people have done on climate change or Jews and Hindus have done over relations between the United States, Israel and India.

“It sets up the potential for big arguments,” Mr. Green said, “but also for the possibility of all sorts of creative synthesis. Diversity cuts both ways.”