For the first time since researchers began tracking the religious identity of Americans, fewer than half said they were Protestants, a steep decline from 40 years ago when Protestant churches claimed the loyalty of more than two-thirds of the population.
A new study released on Tuesday by the Pew Forum on Religion and Public Life found that it was not just liberal mainline Protestants, like Methodists or Episcopalians, who abandoned their faith, but also more conservative evangelical and "born again" Protestants. The losses were among white Protestants, but not among black or minority Protestants, the study found, based on surveys conducted during the summer.
When they leave, instead of switching churches, they join the growing ranks who do not identify with any religion. Nearly one in five Americans say they are atheist, agnostic or "nothing in particular."
This is a significant jump from only five years ago, when adults who claimed "no religion" made up about 15 percent of the population. It is a seismic shift from 40 years ago, when about 7 percent of American adults said they had no religious affiliation.
Now, more than one-third of those ages 18 to 22 are religiously unaffiliated. These "younger millennials" are replacing older generations who remained far more involved with religion throughout their lives.
"We really haven't seen anything like this before," said Gregory A. Smith, a senior researcher with the Pew Forum. "Even when the baby boomers came of age in the early '70s, they were half as likely to be unaffiliated as compared with young people today."
The "Nones," as they are called, now make up the nation's second-largest religious grouping. The largest single faith group is Catholics, who make up about 22 percent of the population. Their numbers have held steady, mostly because an influx of immigrants has replaced the many Catholics who were raised in the church and left in the last five years, Mr. Smith said.
The rise in people who claim no religion is likely to have political consequences, said Phil Zuckerman, a professor of sociology and secular studies at Pitzer College in Southern California.
"The significant majority of the religiously unaffiliated tend to be left-leaning, tend to support the Democratic Party, support gay marriage and environmental causes," he said.
The Pew report offers several theories to explain the rise of the religiously unaffiliated. One theory is that the young adults grew disillusioned with organized religion when evangelical Protestant and Catholic churches became so active in conservative political causes, like opposition to homosexuality and abortion.
Another theory is that the shift merely reflects a broader trend away from social and community involvement, the phenomenon dubbed "bowling alone" by Robert D. Putnam, a public policy professor at Harvard University.
Another explanation is that the United States is simply following the trend toward secularization already seen in many economically developed countries, like Australia and Canada and some in Europe.
The United States has always been the great exception to this secularizing trend, and it is not clear that Americans are necessarily moving toward the European model.
The Pew report found that even among Americans who claimed no religion, few qualified as purely secular. Two-thirds say they still believe in God, and one-fifth say they pray every day. Only 12 percent of the religiously unaffiliated group said they were atheists and 17 percent agnostic.
The Rev. Eileen W. Lindner, who has chronicled religious statistics for years as the editor of the Yearbook of American and Canadian Churches, has observed this complexity.
She said, "There will be lots of people who read this study and go: 'Oh no, this is terrible! What's it doing to our culture?' I would, as a social scientist and a pastor, urge caution.
"A lot of the younger people are very spotty in their attendance at worship, but if we have a mission project, they're here," said Ms. Lindner, the pastor of a Presbyterian church in New Jersey. "They run the soup kitchens, they build the houses in Habitat for Humanity."
They may not come on Sundays, she said, but they have not abandoned their faith.
This article originally appeared in The New York Times.