There is a discussion on Uncommon Descent based on the premise that atheism is a religion. Personally, I hate that argument, and I especially dislike basing important debates on such a weak foundation. It atheism is a religion, then the word "religion" has no real meaning. It simply means: everyone believes something, and whatever it is, it's their religion. It means the same as philosophy or world view. A working definition for religion is: religion is whatever atheism/agnosticism isn't.
Atheism isn't religion. It's closer to being an antonym.
At a minimum, a religion should include some aspect of the supernatural.