RONNELLE ADAMS came out to his mother twice, first about his homosexuality, then about his atheism.

“My mother is very devout,” said Mr. Adams, 30, a Washington resident who has published an atheist children’s book, “Aching and Praying,” but who in high school considered becoming a Baptist preacher. “She started telling me her issues with homosexuality, which were, of course, Biblical,” he said. “ ‘I just don’t care what the Bible says about that,’ I told her, and she asked why. ‘I don’t believe that stuff anymore.’ It got silent. She was distraught. She told me she was more bothered by that than the revelation I was gay.”

This was in 2000, and Mr. Adams did not meet another black atheist in Washington until 2009, when he found the Facebook group called Black Atheists, which immediately struck a chord. “I felt like, ‘100 black atheists? Wow!’ ” he said.

In the two years since, Black Atheists has grown to 879 members from that initial 100, YouTube confessionals have attracted thousands, blogs like “Godless and Black” have gained followings, and hundreds more have joined Facebook groups like Black Atheist Alliance (524 members) to share their struggles with “coming out” about their atheism.

Feeling isolated from religious friends and families and excluded from what it means to be African-American, people turn to these sites to seek out advice and understanding, with some of them even finding a date. And having benefited from the momentum online, organizations like African Americans for Humanism and Center for Inquiry-Harlem have well-attended meet-up groups, and others like Black Atheists of America and Black Nonbelievers have been founded.