Only religious people say atheism is a religion, it's not. It's simply a non-belief in supreme entities. That is the single thing all atheists share.
Actually, it is quite a bit more then that. It is a challenge to the a prior claim of a competing belief system.
Atheism is always accompanied by an often unstated supplementary set of assumptions or faith. The characterization of it as a religion is defensible on logical grounds.
Again, only religious people feel the need to categorise atheism as a religion when the only thing that makes someone an atheist is their non-belief of a supreme entity.
There are no defined set of rules that atheists have to live by, nor specific places, people or entities they have to worship, nor things they have to believe for whatever reason, so there are none of the defining elements to make atheism a religion.
If I, an atheist, were to wake up tomorrow and decided that I now believe in the presence of a supreme entity, would that mean I now instantly have to follow a particular set of rules that I have to live my life by? If so, which religion have I instantly become to know what rules I should be following? And if my religion is suddenly dictated by which entity I believe in, what would happen if I believe in more than a single supreme entity?
My point is that you can both believe or not believe in a supreme entity and still not have a religion. It's only narrow mindedness that creates the need to categorise everyone into a religion for reasons that I purely feel are self serving.
There is only one reason why you are an atheist... only one. Because you say it. That's all. Not because you practice atheism. Not because you believe you are an atheist.
Check out #6 of religion at Dictionary.com - http://www.dictionary.com/browse/religion?s=t. Religion fits everybody.
