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.