Uh, atheism is NOT a religion, much like someone who doesn't collect stamps cannot say that "not collecting stamps" is his hobby.
Atheists don't go to churches, don't meet in groups, don't try to convert others, or do those things normally expected when belonging to a social group. There's no one to baptize you into atheism, no one to DF you, no bodies to certify you as a member, no heads to count, etc.
Most who claim to be either atheists or agnostics don't even understand the difference, since there's no agreement on meaning of these words (the often-heard line is the difference between atheists and agnostics is what you "believe" vs "know", a distinction that is highly-qestionable, IMO: belief and knowledge are so interlinked, intertwined, that it's impossible to tease apart ideas (knowledge) from what you believe: it's a tangled Gordian knot. Further, most people act on reasons which operate on the sub-conscious level, so quite separate from knowledge or beliefs.)
I'm surprised no one has brought up the Pew Study from last year which found that atheists and agnostics have a greater knowledge of others' religious beliefs than most who profess to belong to one of the major faiths:
http://articles.latimes.com/2010/sep/28/nation/la-na-religion-survey-20100928
Of course, ignorance is par for the course, esp when you consider that more people can name the Three Stooges than can name the three branches of U.S. gov't.