Atheism is not a coherent philosophical position - most intellectual Atheists, like Bertrand Russell, would philosophically call themselves agnostic. The position of Ontological Materialism is hard to hold, so most opt for it only as a pragmatic approach to justify their Atheism otherwise.
So Atheism is either an Ideology or a Paradigm of looking at the world, I think. I wouldn't call it a philosophy nor a religion in a strict sense of those words. It does have many similarities though, in that holding it is coupled to confirmation biases and knee-jerk psychologic defensive mechanisms and such.