MIAMI — There's nothing unusual about churches advertising Sunday services, but some atheists are turning that idea on its head: Why not promote the belief that there is no God?

"Most people are under the impression that atheists lack morals and ethics. We are trying to dispel that myth," said Ken Loukinen, founder of the 400-member Florida Atheists and Secular Humanists, which is sponsoring a controversial billboard in Broward County, Fla.

"Being a good person doesn't require God," the sign declares. "Don't believe in God? You're not alone!"

The first of its kind in the state, the sign directs passers-by to www.freethoughtflorida.com, where they can donate toward putting up similar signs throughout Florida. Over the past six months, atheists in a dozen other states also have launched advertising campaigns.

In a bid for greater acceptance and visibility, atheists are also undertaking community service projects, organizing children's camps and engaging in other activities often associated with religious groups.

Best-selling books like Richard Dawkins' "The God Delusion" and Christopher Hitchens' "God is Not Great" have popularized the idea of atheism as a cause rather than simply a stance. And polls show that fewer Americans are embracing religion.

According to the American Religious Identification Survey, a major study released this year, 15 percent of Americans claim no religion, making them the only group to grow in every state since 1990, when the "nones" made up 8 percent of the U.S. population. Atheists make up a smaller portion — 2 percent — but they've almost doubled their numbers in the past two decades.

"Many people would admit to not knowing the answers, but fewer would say they're atheist," says Lesley Northup, a professor at Florida International University who studies religious trends. "People often say 'I'm spiritual, but not religious.' There is a growth of people who say 'I don't need to be religious to be a good person.' "

The billboard may be working: About 30 people attended a recent Florida Atheists and Secular Humanists' get-together at a Davie, Fla., restaurant, up from the usual dozen.

Over beers and burgers, nonbelievers, agnostics, skeptics and lapsed Jews, Christians and Muslims talked about everything from science and philosophy to politics and current events.

The feeling, many said, was liberating: being able to speak casually and openly with like-minded people without fear of being disliked or distrusted.

"I still go to the mosque every once in a while because I'm under pressure to, but usually I tell my Muslim brothers that I pray at home," said Yahya, a 65-year-old former Muslim. "I don't want to pretend to be a believer, but I have to because I like some of these people."

Across the table, Matt Kirsch, a 30-year-old computer engineer and ex-Mormon, proposed that the group collect money to distribute care packages to the homeless.

"We can put together some nonperishable food and a list of helpful organizations and maybe add an inspirational message," he said.

"Like John 3:16?" quipped Loukinen to a round of chuckles.

Groups like Loukinen's object to what they see as an increasing blur between church and state. Florida Atheists and Secular Humanists protested the Broward Sheriff Office's National Day of Prayer observance in May and in February it hosted a Darwin Day family event to celebrate the 200th birthday of the father of evolution.

"Atheists groups are popping up all over the place. I get calls every week from people trying to form groups," said Blair Scott, a national director for American Atheists, a Cranford, N.J.-based umbrella organization. "Fellowship is not a religious word."