Getting all technical here, religion is a belief in a higher being. Therefore, atheism can't be a religion because it does not involve a higher being. However, atheism can be a belief system. Because you can't prove that there is no god, anyone who states that god does not exist is doing so without evidence--it's a belief.Since the existence of God can neither be proven nor disproven scientifically (yet), it comes down to a matter of faith on either side of the same coin on what one believes to be true. Hence, religion, in the strictest sense of the term.
There is a difference between a religious belief and a more general belief. Atheism can be a belief, it can be dogmatic, but it can't be a religious belief.