I am not an Atheist, however I do know a great many who would be better represented (at least in their minds) with a slightly different yet somewhat related answer.

"Reason"

This is the "credo", so to speak, of the intellectual Atheist. I am forced to admit that it is what I claim as the capital cause of my Agnostic lack of belief.
It would also be noteworthy to point out that "Reason" is also claimed by many modern minded christians as the very spark of their faith. Objectively speaking, these are starkly contrary. Deeper still, down the well of logic, I find that we may all be better served if we swapped out "Reason" with "Will" or "Choice" as it is quite a bit more base, honest and more easily defended. :P