At least from my (personal) perspective, the lack of belief in gods stems from not having found or experienced sufficient reason to think there likely are gods. It's not that I'm claiming to know or believe that there are certainly not any gods out there in the universe. I just can't find anything to back up the claim that there are.
But isn't that more agnosticism that atheism?
Gnosticism and agnosticism refer to knowledge or lack thereof. If I cared to use labels, I'd classify myself as an agnostic atheist. I don't claim the certainty of knowledge that there couldn't be deities, but I don't personally believe there are any (at least not in the sense of ultra-powerful entities that have/had some sort of interaction with the doings of humanity).
1) Either faith can be used in an argument, or it can't
2) If it cannot, then it cannot.
3) If it can, then I have faith that it cannot.
4) 3 contradicts itself so we are left with its alternative 2.
What is the basis of atheism? Is it evidence based? Is is gut feeling based? What the general or most commonly cited bases of the validity of atheism by atheists?