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 5 percent to 8 percent of the population described itself as unaffiliated with a particular religion.  The Pew survey, available on the Web at http://religions.pewforum.org/, was conducted between May and August of 2007. The margin of sampling error ranges from plus or minus one percentage point for the total sample to two points for Catholics and eight points for Hindus.  In the Pew survey 7 percent of the adult population said they were unaffiliated with a faith as children. That segment increases to 16 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, however, mean that Americans are becoming less religious. Contrary to assumptions that most of the unaffiliated are atheists or agnostics, most described their religion “as nothing in particular.” Pew researchers said later projects would delve more deeply into their beliefs and practices and would try to determine if the unaffiliated remained so as they aged.  The other groups that have gained the most people, in net terms, are nondenominational Protestant churches, which are largely evangelical and, in many cases, megachurches; Pentecostals; and the Holiness Church, also an evangelical denomination.  While the ranks of the unaffiliated have been growing, Protestantism has been declining, the survey found. In the 1970s, Protestants accounted for some two-thirds of the population. The Pew survey found they now make up about 50 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.  Prof. Stephen Prothero, chairman of the religion department at Boston University, said large numbers of Americans leaving organized religion and large numbers still embracing the fervor of evangelical Christianity pointed to the same desires.  “The trend is towards more personal religion, and evangelicals offer that,” Professor Prothero said, explaining that evangelical churches tailored much of their activities to youths.  “Those losing out are offering impersonal religion,” he said, “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 Roman 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 as 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,” said Mr. Green of Pew, “but also for the possibility of all sorts of creative synthesis. Diversity cuts both ways.”  A version of this article appears in print on , on Page A12 of the New York edition with the headline: A Fluid Religious Life Is Seen Among Americans. Order Reprints| Today's Paper|Subscribe
More than a quarter of adult Americans have left their childhood faith for another religion or no religion.