When Greg Epstein arrived as humanist chaplain at Harvard University four years ago, just a handful of students would show up at his events, intended for nonreligious young people looking for a values system and a sense of community. The school's humanist chaplain, first installed on campus in the 1970s, had always taken what Epstein calls a "shy, retiring approach to his presence on campus."

But not Epstein. "The watchword now for young humanists and nonreligious communities like mine around the country is that we're going to be loud and proud," he says. "The time has come to recognize that whatever you choose to call us nonreligious people, we are an integral part of society and culture."

The message has found a receptive audience. All 1,000 tickets to Harvard's big annual event for secular humanists next month—a ceremony to present a humanist lifetime achievement award—have already been sold. And Epstein can't schedule more events quickly enough. "An enormous number of young people have left traditional religion behind," he says. "I'm overwhelmed by the number of people coming to me."

The phenomenon is hardly limited to the elite confines of Harvard Square. In fact, Epstein and the nonreligious students he leads are part of the fastest-growing demographic on the American religious landscape: those who claim no religion whatsoever.

According to a comprehensive national survey released this week by the Program on Public Values at Trinity College, those identifying with no religious tradition, or as atheists or agnostics, account for 15 percent of the population, up from about 8 percent in 1990. "No religion" Americans are the only religious demographic that's growing in every single state.

And in northeastern states like Massachusetts, New Hampshire, Vermont, and Maine, the "no religion" group has surged even more dramatically, shooting up 300 percent in the last 20 years and now accounting for a quarter or more of the population.

With as many Americans identifying as "no religion" as there are mainline Christians, Jews, and Mormons in the United States combined, the Trinity College survey has helped create a portrait of an exploding secular tradition that reveals commonalities beyond lack of belief.

It turns out that Harvard's Epstein, 32, is pretty typical. The Trinity College report, called the American Religious Identification Survey, finds that 60 percent of the nonreligious are men. They tend to be young, accounting for one in every three American adults under age 35. According to Trinity College Professor Barry Kosmin, a large chunk have baby boomer parents who came of age in the 1960s and wound up rejecting religion.

And Kosmin says that many of the 750,000 additional American adults who each year identify as having "no religion" are reacting to what he calls the "triumphalism and judgementalism of the Christian right."

A full quarter of those identifying as "no religion" in the Trinity College report are former Catholics, many of whom were turned off by the church sex abuse scandals of the past decade. That helps explain why the Northeast now rivals the Mountain states and the Pacific Northwest—whose frontier beginnings established rugged individualist traditions that resisted organized religions—as the most secular parts of the country. "Despite the population growth, New England has lost 1 million Catholics" in the last decade, says Kosmin. "The trend in the Catholic Church has been obscured by the large number of people from Latin America who've filled the pews as the Irish Americans left them."

Other religious traditions feeding the "no religion" boom are Judaism and Asian religions like Buddhism and Hinduism. While people who leave mainline Protestant churches often find new spiritual homes in evangelical or nondenominational megachurches, the Trinity survey shows that former Catholics, Jews, Buddhists, and Hindus are much more likely to abandon religion altogether. Nearly half of "no religion" Americans come from Irish, Jewish, or Asian backgrounds.

Only a small minority of the nonreligious call themselves atheists or agnostics, but just 21 percent believe in a personal God, compared with 70 percent of all Americans. "They're the inverse of the rest of the American population," says Kosmin. "Three in four Americans want a religious funeral, but in this group, three in four don't."

The growth of nonreligious America has obvious implications for the religious traditions that the nonbelievers are leaving and also for the public square, where religious Americans have tried to reassert their influence in recent decades. Roughly half of "no religion" Americans are political independents, with many fleeing the GOP since the rise of the religious right. About 30 percent are Democrats, while just 12 percent are Republicans.

As their numbers grow and the stigma of being nonreligious fades, nonbelievers are beginning to raise their voices to combat the influence of religion in politics. "I wouldn't have even called it a movement up to a few years ago, but more a club of the like-minded," says David Niose, president of the American Humanist Association. "But now we're unifying people who are secular and humanists into a lot more of a defined demographic. We want a place at the table."

Read more by Dan Gilgoff.