The word atheism denotes nothing but a rejection of a belief and a concept. That is often why the word atheism is so problematic, because it is merely one belief, not a set of beliefs. If someone tells me that they are Christian, I can access my knowledge of many Christian beliefs because Christianity is almost a complete belief system. If someone tells me that they believe in God, that is, that they are a theist, this is only one belief and not a set of beliefs. Yet we are still able to assume more things about theism than atheism.
Theism is a positive belief.
I don't mean positive in the sense of being optimistic, but in that it posits something. The point is that this positive belief cues us off to the belief in the existence of something. Whereas theism posits the thesis that God exists, the atheism denies the thesis. Atheism as a term says less and commits to less conceptually and linguistically than theism does. To illustrate my point, imagine what it would be like if we were all religiously labeled not by what we believed in, but by what we did not believe in.