What you fail to understand is that atheism is not itself a belief system. It is the absence of belief. What you're doing is playing with words because you can't make a logical argument.
I'll make this simple for you- my brain is capable of comprehending the concept of gods and I "know" of gods because other humans have imagined them. But I think the concept is silly and physically impossible in our NATURAL universe, so I don't believe gods exist but in the minds of men. Similarly, people imagined Santa Clause and kids believe in him. I don't. Do you?