The study also found that most traditional Christians reject the label "evangelical," preferring to describe themselves as "Bible-believing" or "born again."
The survey was conducted by the Baylor University Sociology Department and the Baylor Institute for Studies of Religion as the first in a series on the spiritual life of Americans.
RELIGIOUS BELIEFS: How the USA views God
Researchers found that only 10.8% of Americans have no ties to a congregation, denomination or faith group. Previous surveys had put that figure at 14%, overlooking about 10 million people involved in some form of organized religion, the Baylor report said.
Other surveys have also overlooked millions of evangelicals, because respondents who belonged to non-denominational groups or megachurches would often report that they had no denomination and were wrongly counted as unaffiliated, the study's authors say.
Baylor researchers found that one-third of Americans are evangelical Protestant, just under one-quarter are mainline Protestant, one-fifth are Roman Catholic and 5% are black Protestant. Jews compromise 2.5% of the population, while 5% of Americans belong to other faiths.
The rest, who are not involved in religious groups, are not fully secular, researchers said. More than 60% of the unaffiliated say they believe in God or a higher power, and nearly one-third say they pray at least occasionally. Eleven percent believe Jesus is the son of God.
Among the more religiously observant Christians, the term "evangelical" is unpopular, according to the study. Nearly 70% of evangelical and black Protestants say "Bible-believing" better describes their views. Nearly as many liked the term "born-again."
Only 15% of all respondents called themselves "evangelical" and within that group just 2% said it was the best description.
The study also looked at the market for religious goods, including books and movies.
One-fifth of respondents have read either The Purpose Driven Life by pastor Rick Warren or the Left Behind series of apocalyptic novels, the survey found.
Yet, even more — 28.5% — had read The Da Vinci Code, the best-selling mystery novel that Christians condemned as an affront to their faith. Still, the study found that the book had little impact on churchgoers.
Asked whether God favored the United States, only one-fifth of respondents said they agreed. Evangelical Protestants were the most likely to agree, with 26% saying they think God favors the country.
Researchers also examined Americans' conception of God and found the greatest share — about 31% — think of God as "authoritarian," deeply involved in people's lives and world events, angry and capable of punishing those who are unfaithful.
Nearly one-quarter consider God a "distant" force that set the laws of nature in motion, but is not active in the world, the study found. About the same percentage view God as "benevolent," active in their daily lives, but less willing to condemn or punish.
And about 16% consider God "critical," an observer who views the state of the world unfavorably and will mete out punishment in another life.
The study also asked respondents about paranormal beliefs such as whether houses can be haunted or whether people can communicate with the dead. The report found that these beliefs are more prevalent in Eastern states.
The survey of 1,721 respondents has a margin of error of plus or minus 4 percentage points and was conducted by the Gallup Organization between Oct. 8 and Dec. 12, 2005.