More than one-fourth of U.S. adults have made a leap of faith from one religious tradition to another, and those who belong to no religion have increased dramatically, according to a new study by the Pew Forum on Religion and Public Life.

"This report shows that religious affiliation in the United States is extraordinarily diverse and dynamic," said John Green, senior fellow at Pew.

More than 36,000 adults answered 45 questions from the nonpartisan organization, resulting in a highly detailed study. Surveys are crucial because the U.S. Census has not allowed questions on religion since 1957.

Of an adult population of 225 million, 28 percent have changed traditions, perhaps moving from Catholic to Protestant or from unaffiliated to Jewish. If people who have swapped within Protestantism are added, 44 percent have changed.

The category that had gained the most from switching was "unaffiliated," which means adults who are not currently affiliated with any religious group. That category has grown to more than 16 percent of Americans. But these non-joiners are not necessarily hostile to faith. About 5 million people -- fewer than 2 percent of adults -- claim to be atheists and about 6 million -- 2.4 percent of adults -- call themselves agnostic. More than a third of the unaffiliated consider themselves religious.

The number of atheists and agnostics has remained steady.

"What we see is a lot of interest in books by atheists and about atheism, but not necessarily a dramatic jump in the number of people who identify that way," Dr. Green said.

The report did not identify why people change traditions. But researchers were surprised to find it was rarely due to marriage. People who have changed affiliation are much less likely to be married to someone of their own tradition than those who have not changed.

Overall, more than 74 percent of Americans are Christian, with once-dominant Protestants hanging on to 51 percent of the population. About 26 percent of Protestant adults belong to white evangelical churches, slightly outnumbering the 24 percent who are Catholic. About 18 percent belong to mainline Protestant churches -- this report did not count evangelicals in those churches -- and 7 percent belong to historically black churches.

Mormons and Jews are tied at just under 2 percent of the population, Muslims are less than 1 percent, as are Orthodox Christians, Buddhists, Hindus and Unitarians.

Immigration has allowed the Catholic Church to hold steady as the nation's largest denomination, but about 10 percent of Americans are ex-Catholics. Catholic losses are minor compared to the Jehovah's Witnesses, who lost nearly two-thirds of their youth.

The unaffiliated also often fail to pass along their beliefs. More than half of those who were raised without any affiliation now belong to a faith.

But David DeAngelo, 44, a paralegal from Shadyside, has adopted his parents' lack of faith as his own. They had him baptized Catholic but never took him to church. In college he attended a liberal Congregational church, then became a Buddhist. (This makes him a poster child for two other study findings: Most U.S. Buddhists are white converts, and half of all Buddhists leave the faith.)

He ultimately decided that it is foolish to believe what cannot be proven. If pressed, he calls himself an agnostic. He spends Sunday mornings working on his blog, "2 Political Junkies."

He's content in his unbelief, and accepting of others.

"I'm not among those who treat believers with any sort of scorn or pity. Even if I don't believe what they believe, I have to respect the depth of feeling that they have," he said.

The Rev. Donald Green, executive director of Christian Associates of Southwestern Pennsylvania, said the study confirmed trends that have challenged clergy for 25 years.

"People are not brand loyal but are demonstrating consumerism, going where their felt needs are addressed rather than seeking out a particular tradition," he said.

Catholics are 24 percent nationally and 29 percent statewide, but close to 40 percent in southwestern Pennsylvania, he said. He also believes that the number of Orthodox Christians in this region may be as high as 2 percent, compared to less than 1 percent nationally.

Protestants were once the dominant religious force in America, but they are barely a majority today. Evangelical Protestant churches are growing, but not enough to make up for mainline losses, Dr. Green said. That partly reflects low mainline Protestant birthrates. But when evangelicals change denominations, it's usually to another evangelical denomination, while many former mainliners join the evangelicals or the unaffiliated.

Nondenominational evangelical churches have experienced the second highest growth rate after the unaffiliated.

Catholics lose about 30 percent of those born into the tradition, a rate comparable to that of Mormons, and less than that of Jehovah's Witnesses, Buddhists and the unaffiliated. But they remain about one-fourth of the population because of immigration. About 70 percent of immigrants to the U.S. are Christian, and Catholics outnumber Protestants 2-1, the study found.

But nearly 3 percent of Americans have converted to Catholicism. Bill Hill, 63, who works in media relations for the Catholic Diocese of Pittsburgh, was raised Methodist in a tiny Alabama community where there was no Catholic Church. He knew nothing about the faith until he met his wife, Barbara.

When they married in 1967, non-Catholics were still required to promise to raise the children Catholic. He agreed, and accompanied his family to church for 13 years. His family made no effort to convert him, and he didn't convert for them, he said.

He was impressed by the sacramental nature of Catholicism, by how it offered spiritual help in every season and crisis of life. In 1980, "I told the pastor that I had been auditing the course long enough, I wanted to take it for credit," he said.

The study found that black Americans are most likely to have a religious affiliation. One surprise of the survey was that in a major historically black denomination, the Church of God in Christ, more than 10 percent of members are white and another 13 percent are Latino.

Barbara Brewton has been a member of Community Friendship Church for 26 years. The interracial church in Oakland is part of the Presbyterian Church (USA), and its membership includes people formerly from all faiths, she said. For Ms. Brewton, who was raised Catholic, "it took getting used to."

But, she said, "For me, the core was the relationship with the people in the congregation."

The church's 75 members have a "shared sense of calling to be deliberate about racial reconciliation," Ms. Brewton said.

"That is definitely fundamental to who we are."

Dr. Green said the study provides a snapshot of the present but cannot foretell the future.

"It's difficult to predict where America will end up 10 to 20 years from now, but we can say it will be more diverse," he said.