Really? Is it?......Theists always bring this up during debates and discussion for whatever reason..I don't know. However, this is a silly notion as atheism is neither a belief or a religion but LACK of belief and LACK of religion. The prefix "A" before theists should be clearly seen as the absence of something, but then again theists don't use common sense.
The Merriam-websters dictionary defines religion as; "(1) The service and worship of God or the supernatural (2) : commitment or devotion to religious faith or observance" and those are just 2 of the many definitions I can pick out for the term religion. Now neither of those apply to atheism. There is no service and worship of anything, no devotion to any religious faith.
Theists might argue however, that atheism is a community and we have a set of beliefs and attitudes. I agree, but does that make it a religion? Then going by that logic, drinking beer on a friday night is a religion.