LYNN NEARY: Now, our Cover Story on the estimated 4 percent of Americans who, according to the Princeton Religious Research Center, call themselves atheist, or nonbelievers in God or a universal spirit. We know what they do not believe in, but what does inform their beliefs, and how do their communities regard them? Betty Rollin of NBC News recently visited a gathering of atheists on the 4th of July.

BETTY ROLLIN: Talladega, Alabama, the heart of the Bible belt. Looks like a church picnic, but it doesn't sound like one.

Unidentified Man #1: Come on, you atheists, let's see what you can do.

Unidentified Man #2 (Singing): I don't need Jesus to give me a smile.

Professor DELOS MCKOWN: You have the descent into hell, and you have the Resurrection, and you have the Ascension, and you have the Second Coming. These people think they're talking about something. They really, really do.

ROLLIN: It's the annual gathering of the Freedom from Religion Foundation, a national organization with about 4,000 members, hosted by its local chapter, the Alabama Free Thought Association. Of the 200 gathered here, many say they were good Christians once. Dan Barker used to preach the gospel until his second conversion took hold.

DAN BARKER (Former Christian Minister): But in my early 30s, I discovered this hunger to know what I was preaching about. So I studied. I read the scholars -- the Christian scholars. And as I learned more, I realized that a lot of the basis that I was preaching had no base at all. So I migrated across, to fundamentalist to more of a loving evangelical Christian to more of a moderate Christian, eventually, after a few years, to become a liberal Christian [of the kind] that I used to preach against. And finally, I dumped out all the bathwater, and I found there's no baby there, and there's no basis for what I was preaching. And I had to be honest and ask myself, do I want God, or do I want the truth? And I went with the truth, and I became an atheist.

ROLLIN: Although most remain invisible, more than a million Americans call themselves atheists. Some of them even come in families. Meet the Warners, from Valley Park, Missouri. Bill is an engineer; Ann, a schoolteacher.

BILL WARNER (Atheist): I don't believe in a supernatural power because I don't see any evidence for it. I look upon it as our current mythology.

ANN WARNER (Atheist): I believe that you don't have to go to church to be a good person. And that's the way we're raising our children.

CHRISTINE WARNER (Atheist): My one best friend in my school, she already knows I'm an atheist, and she doesn't really care that much.

ANTHONY WARNER (Atheist): Ryan, he always says if you don't believe in God, you'll go to the devil.

ROLLIN (To A. Warner): Somebody said that to you?

A. WARNER: Ryan did.

ROLLIN (To A. Warner): And what did you say? What do you think about that?

A. WARNER: I said, who's the devil?

ROLLIN: But for some families, being criticized for atheism is no joke.

Unidentified Man #3: People are very violent about it in the rural areas.

ROLLIN: So violent that this man says he doesn't dare show his face. His wife once admitted they were nonbelievers to a neighbor.

Man #3: Her daughter found out and told everybody at school the next day, and my daughter was attacked on the bus.