Right. It would be impossible for anyone to think 'X doesn't exist' is they didn't have the concept 'X'. So there is a sense in which atheism is kind of logically dependent on theism, since theism is the source of the concept of "God" whose existence atheists don't believe in. In other words, if the theists had never produced the "God" idea, then I'd never have reached my own conclusion that the existence of this "God" is highly unlikely. I wouldn't have any ideas about "God" at all. All that I could do in that case would be to say that the category of 'the unknown' contains a large but undetermined number of possible concepts that nobody has thought of yet, and that some of those concepts will turn out to have real existing referrants and some won't.