The Roman Catholic Church has lost more adherents than any other group: About a third of respondents who were raised Catholic say they no longer identify as such. The group that shows the greatest gain is the unaffiliated.
The survey, conducted by the Pew Forum on Religion and Public Life, is particularly important because the U.S. Census does not track religious affiliation.
"Religion is the single most important factor that drives American belief, attitudes, and behaviors," says Michael Lindsay, a sociologist at Rice University in Houston. "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."
Detailing the nature of religious affiliationwhich groups are the largest, the best educated, and the most affluentcan signal who holds sway over the country's political and cultural life, says John Green, one of the authors of the Pew report.
In fact, the United States is unusual among industrialized democracies for how religious it remains. It's generally the case that as nations become more prosperous, healthy, and educated, demand for the support that religion provides declines.
The industrial democracies in Asia and Europe bear this out. According to the Pew Global Attitudes Project, only 20 percent of Germans, 12 percent of Japanese, and 11 percent of the French say religion plays a very important role in their lives.
But in the U.S., religious expression seems to have grown, not diminished, with socioeconomic development. According to Roger Finke, a sociologist at Pennsylvania State University, 45 percent of Americans were members of a church in 1890. By 2000, that figure was 62 percent.
Some sociologists have a novel explanation for why the U.S. hasn't followed this pattern: supply-side economics. Americans, they say, are more religious because there are so many churches competing for their devotion and finding ways to be more responsive to their needsunlike in Europe, where there tend to be official state churches that in some cases had monopolies on religious life for centuries.
The supply-side theory is consistent with the fluid and diverse American religious life that the Pew survey found. Including shifts among Protestant denominations, 44 percent of Americans have switched from the religion they grew up with.
The Pew survey, which was based on phone interviews with more than 35,000 Americans, shows that the percentage of Catholics in the U.S. has held steady for decades at about 25 percentlargely because of the influx of Catholic immigrants, mostly from Latin America. That has made up for a precipitous decline in native-born Catholics, the Irish-Americans and Italian-Americans who once dominated the church.
Immigration continues to influence American religion greatly, the survey indicates. 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, according to the survey.
"I think politicians will be looking at this survey to see what groups they ought to target," says Stephen Prothero, chairman of the religion department at Boston University. "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."
Other interesting findings are that 37 percent of the married respondents have spouses from a different religious tradition, and that 16 percent of adults say they are not part of any organized faith, which makes "unaffiliated" the country's third-largest "religious group" (see graph below).
That's a substantial increase from the 5 to 8 percent of the population that described itself as unaffiliated with a particular religion during the 1980s, according to a survey done then by the National Opinion Research Center. The rise of the unaffiliated does not, however, mean that Americans are becoming less religious. Most in this group described their religion "as nothing in particular."
"It wouldn't be accurate to think of this group as just secular," says Greg Smith, another one of the Pew report's authors. "Just because you're unaffiliated doesn't mean you're irreligious."
The converse is also true, Smith adds: The study shows a significant number of people who identify themselves as belonging to a particular religious group but also say that religion is not important to them.
The other groups that have gained the most people are nondenominational Protestant churches, which are largely evangelical and, in many cases, so-called mega-churches with congregations upward of several thousand people. At the same time, mainline Protestant denominations, such as Episcopalians and Presbyterians, have been declining.
Experts say 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. "It sets up the potential for big arguments," says Green of Pew, "but also for the possibility of all sorts of creative synthesis. Diversity cuts both ways."