A major new survey of the religious leanings of American adults has found that the country’s spiritual landscape continues to shift — with barely half of adult Americans identifying themselves as Protestants and with 16.1 percent claiming no religious affiliation at all.

The U.S. Religious Landscape Survey found that for many Americans, religious affiliation is anything but permanent. More than a quarter of American adults (28 percent) have left the faith tradition in which they were raised, switching to another religious tradition or to no affiliation at all. If switching from one stripe of Protestantism to another also is counted, 44 percent of American adults have either changed their religious tradition, gone from no faith tradition to choosing one, or dropped any affiliation at all.

About a quarter of American adults (26.3 percent) consider themselves evangelical Protestants and another quarter (23.9 percent) are Catholics.

The survey also found that the American religious picture is becoming increasingly diverse, in part because of immigration, variations in birth rates, and conversions from one religious tradition to another.

“American religion is likely to be even more diverse in the future than it is now,” said John Green, senior fellow for the Pew Forum on Religion & Public Life, which released the survey. “Where exactly we will come down is hard to say,” but he said he expects to see “a less Protestant and a less Christian nation” a century from now and “it will be more diverse, and diverse in ways we can’t anticipate.”

The telephone survey of more than 35,000 American adults between May and August 2007 is considered significant in part because the large number of people surveyed allows the researchers to measure religious affiliation in even relatively small groups — as small as three-tenths of 1 percent.

The first set of results released looks at religious affiliation. Future reports, with the next expected to be issued this spring, will examine spiritual practice, and social and political views.

Here are some key findings from the survey.

Protestants. As recently as the mid-1980s, Protestants accounted for about two-thirds of the American population. But in this survey, just barely over half of adult Americans — 51 percent — identified themselves as members of Protestant denominations. 

About a quarter of those surveyed (26.3 percent) identified themselves as evangelical Christians and only 18.1 percent as members of mainline Protestant denominations. Another 6.9 percent were affiliated with historically black denominations.

“The United States is on the verge of becoming a minority Protestant country,” the report states. 

Catholics. Catholicism has experienced the greatest net losses as a result of changes in affiliation, but continues to experience growth through immigration. Nearly a quarter of adult Americans (23.9 percent) currently identify themselves as Catholic – but nearly one in three (31 percent), were raised as children in the Catholic faith, which means that many have left.

“These losses would have been even more pronounced if it were not for the offsetting impact of immigration,” the report states. Among foreign-born adults surveyed, Catholics outnumbered Protestants 2 to 1 (with 46 percent of the immigrants being Catholic and 24 percent Protestant).

About one in three adult Catholics in the U.S. is Latino – including nearly half of all Catholics (45 percent) of those ages 18 to 29.

Unaffiliated. Those not affiliated with any religion (16.1 percent of the overall adult population) have seen the greatest growth as a result of changes in affiliation. People moving into the unaffiliated category outnumber those leaving the unaffiliated category by a three-to-one margin.

Within the “unaffiliated” category, however, the survey found significant diversity – and some evidence of religious belief.

 About one-quarter of the unaffiliated described themselves as atheist (1.6 percent of the overall adult population) or agnostic (2.4 percent). Most of the unaffiliated group (12.1 percent of the overall adult population) described themselves as “nothing in particular” in religious terms.

And the “nothing in particular” group was about evenly divided between what the report called the “secular unaffiliated,” who said religion was not important in their lives (6.3 percent of the overall adult population) and the “religious affiliated,” who said religion was either somewhat important or very important in their lives (5.8 percent of the overall adult population).  

Competitive marketplace. The survey found “a very competitive religious marketplace” characterized by “constant movement,” in which “every major religious group is simultaneously gaining and losing adherents.”

Take the unaffiliated group, for example. Eight percent of the overall adult population says they were raised as children outside of any religious tradition. The report says a “substantial number” of those (4 percent of the overall adult population – about half of those raised outside of a faith tradition as children) now identifies with a religious group.

But others who did grow up in a religious tradition have dropped that affiliation as adults, so overall, the unaffiliated category is growing, “despite having one of the lowest retention rates of all ‘religious’ groups,” the report states.

Other surveys have found that the percentage of Americans who are Catholic has held fairly steady over the last three decades, at about a quarter of the American population. But that overall statistic conceals considerable change within the Catholic population. About a third of those who were raised Catholic said in this survey they are no longer Catholic, which means about one in 10 Americans is a former Catholic.

 But the disproportionately high percentages of Catholics among immigrants to the U.S. has helped to offset those losses and hold the overall percentage share of the Catholic population steady.

Growing diversity. Religions other than Christianity account for 4.7 percent of the overall adult population in the survey. That includes Jews (1.7 percent), Buddhists (.7 percent), Muslims (.6 percent), Hindus (.4 percent), Unitarians (.7 percent) and New Age practitioners (.4 percent). Roughly two-third of the Muslims and eight in 10 Hindus are immigrants.

Age. The survey revealed variations based on age that, if they continue over time, could have considerable repercussions in the country’s religious life.

More than six-in-10 Americans age 70 and older (62 percent) are Protestant, but only four-in-10 (43 percent) young adults consider themselves Protestant. 

Only 8 percent of those ages 70 and older say they are not affiliated with any religious tradition. But one-in-four of those ages 18 to 29 say they are not affiliated.

Among mainline Protestants, just 14 percent were ages 18 to 29; more than half were 50 and older.

“If these generational patterns persist, recent declines in the number of Protestants and the growth in the size of the unaffiliated population may continue,” the report states.

Mainline vs. evangelical. The declining share of mainline denomination reflects both lower birthrates and “an inability to keep those born in those churches in those faiths,” said Green, in a telephone news conference.

Also, when evangelicals switch affiliations, they generally move to another evangelical group and when mainliners switch affiliation, they often move to an evangelical church as well.

The continuing decline of mainline denominations “is very important for American culture and American politics,” Green said in a news conference held via conference call. “So much of the values and institutions in American public life came out of Protestantism, particularly mainline Protestantism. … It certainly will be very different than it was in the past.”