Atheism is not a belief system. There are belief systems that are atheistic and there are belief systems that are theistic. For example, Christianity is a theistic religion based of the belief of a magical cloud genie. LeVeyan Satanism is an example of an atheistic religion, based on individualism. So can a person be an atheist? Yes, if their belief system is atheistic, but you shouldn't say that atheism is your religion, just as you wouldn't say your religion is theism.
There, dead horse thoroughly beaten.