Many people make a distinction between "soft" and "hard" atheists.
Basically, atheism is about belief, while agnosticism is about knowledge.
Most atheists appear to be agnostic (soft) atheists. That is to say that they do not believe in god(s) but do not know that there are no gods. If solid, reproducible proof were given, most, if not all, atheists would, of course, believe.
On the other hand, most of us also seem to believe that the lack of evidence is so compelling that the nod to possible gods is a very tiny one, and we will be surprised as all get out if any of the gods we've heard about actually exist.