A common claim of many atheists is that atheism is not a belief, but a lack of one. You can see this claim being debated in the discussion of the article, Atheists: They Exist in Modesto, California! posted on Friendly Atheist.

However, a lack of belief in either a theist or atheist direction is apisticism (a lack of knowledge is agnosticism)—someone who lacks belief about god(s) believes neither that god(s) exists (theism), nor that no god(s) exists (atheism). So to claim the title ‘atheist’ is to believe there are no god(s) (positive belief) at the same time one disbelieves in the existence of god(s) (negative belief). Negative belief is belief nonetheless.