Of course no one can be sure. But then again, as Russell pointed out, one can claim there's a very small tea pot somewhere in space that no one can see and they can't prove there's such a thing, but you just have to accept it faithfully that there is. Atheism is not the state of being sure there is no divine being that's responsible for our existence. It's the state that we need more than just blind faith to accept there is a God and until then, we have no reason to believe there is.