This isn't a huge surprise: In the United States, according to a survey by the Pew Forum on Religion & Public Life, atheists and agnostics are more knowledgeable than Christians about religion, says the Los Angeles Times, followed very closely by Jews and Mormons. Why would that be? The Times quotes a Pew researcher:
American atheists and agnostics tend to be people who grew up in a religious tradition and consciously gave it up, often after a great deal of reflection and study, said Alan Cooperman, associate director for research at the Pew Forum.
"These are people who thought a lot about religion," he said. "They're not indifferent. They care about it."
Atheists and agnostics also tend to be relatively well educated, and the survey found, not surprisingly, that the most knowledgeable people were also the best educated. However, it said that atheists and agnostics also outperformed believers who had a similar level of education.
The New York Times's article has a more-acerbic quote from an atheist:
That finding might surprise some, but not Dave Silverman, president of American Atheists, an advocacy group for nonbelievers that was founded by Madalyn Murray O’Hair.
“I have heard many times that atheists know more about religion than religious people,” Mr. Silverman said. “Atheism is an effect of that knowledge, not a lack of knowledge. I gave a Bible to my daughter. That’s how you make atheists.”
(For more data on educational levels of different categories of believers, see an earlier Pew survey.)