When the Indiana Atheist Bus Campaign and American Humanist Association wanted to spark a public conversation last spring about the origin of religion, they plastered Chicago's buses with a provocative twist on Genesis: "In the beginning, man created God."

When the Chicago Coalition of Reason wanted to proclaim that no one needs God to be good, they posted a billboard a few months later above a LaSalle Street sandwich shop in Chicago's Loop.

So when the Wisconsin-based Freedom From Religion Foundation took its controversial slogan touting the benefits of sleeping in on Sundays to honor the day of rest, they came to Chicago first, a city where they knew they would be welcome.

"There are more freethinkers in Chicago than we have in our membership," said Annie Laurie Gaylor, co-president of the Freedom From Religion Foundation, which rolled out a series of ads on Chicago buses this month. Freethinkers argue that beliefs should be based on rationality, not on religious tradition or dogma.

"We have to reach them, provide an alternative to religion and let them know we're here. Many of us reject the idea of blind faith," Gaylor said.

Historically scorned and forced to live in silence for decades, atheists, agnostics and self-described freethinkers are basking in the glow of a renaissance, spreading their gospel of reason in lieu of religion with billboards, bus placards and celebrity endorsements. Advocates have found particularly fertile ground in the diverse religious landscape of Chicago, where the American rationalist movement took root at the turn of the 20th century.

Their messages vary. While some secularists want to promote an ethic of goodwill toward men and women and show like-minded people that they aren't alone, others want to dish out what they have been taking all these years.

"This is a reaction to the newly emergent political power of Christians," said Phil Zuckerman, professor of sociology at Pitzer College in Claremont, Calif., referring to the religious right movement of the past 30 years. "I think secular people feel they need to respond if they want to shape their own world and society."

The campaigns also signal a demographic shift, said Ariela Keysar, associate director for the Institute for the Study of Secularism in Society and Culture at Trinity College in Hartford, Conn.

"They have a growing audience," she said. "They have people who can not only pay attention but sympathize with that message."

The most recent American Religious Identification Survey, conducted in 2008, found that the "no religion" category was the only group to have grown in the continental United States since the previous poll in 1990.

Nationally, the "nones," as they have been dubbed, nearly doubled, from 8 percent to 15 percent. In Illinois, the "none" population grew from 8 percent to 13 percent. Experts say those numbers don't account for the atheists and agnostics who still stay mum about religion.

In fact, nonbelievers have felt alienated for decades, said Hemant Mehta, coordinator of the Chicago Coalition of Reason, the group that sponsored a billboard last fall that asked passers-by: "Are you good without God? Millions are."

"Our goal was to say 'Hey, atheists out there already, we're here. If you agree with us, come check us out,'" Mehta said. "Meeting people who are like-minded can be really tough for atheists."

Matt Lowry, 37, a science teacher in Vernon Hills and the organizer of the North Suburban Chicago Freethinkers, said he never wore his beliefs, or lack thereof, on his sleeve. But the more it came up in conversation, the more he realized how the collective silence has caused atheists and agnostics to be misunderstood.

"Just because you're a nonbeliever does not mean you're kicking little old ladies down the stairs or eating babies," Lowry said. "There's this common misperception. For too long, the nonbelieving community in this country has basically allowed religious fundamentalists to define them that way."

Though nonbelievers generally eschew dogma, they do abide by a moral code much like the world's major religions. Zuckerman said the golden rule — Do unto others as you would have them do unto you — is the "sum total of atheist morality."

Nonbelievers also generally value independent thinking, freedom of thought, freedom of speech and equality, he said.

It's hard to know if the confrontational way these groups present their message is healthy for the body politic, said the Rev. Paul Rutgers, co-executive director of the Council of Religious Leaders of Metropolitan Chicago.

"Quite obviously a significant portion of the religious community is going to be upset," Rutgers said. "We'll probably do several things, including be angry and deeply offended. Perhaps also some will give more serious thought to their own faith commitments. The fact of the matter is we're living in a culture that in many ways beyond this challenges our faith commitments. So in that regard it's a reminder that in many respects this is not a friendly environment."

mbrachear@tribune.com