Atheism is a very, very narrow thing.

[ffr]

48. Atheism.org definition of atheism (which is the way I've always understood it)

Atheism is not an affirmative belief that there is no god nor does it answer any other question about what a person believes. It is simply a rejection of the assertion that there are gods. Atheism is too often defined incorrectly as a belief system. To be clear: Atheism is not a disbelief in gods or a denial of gods; it is a lack of belief in gods. - Atheism.org

The foundation boils down to one simple problem, synthetic a-priori knowledge, or knowledge before the fact. I express the question simply with the following example. Let's assume there is a deity and a book or bible was written about this deity and their teachings, but one problem, the book was lost after being buried and everyone with knowledge of the book and its whereabouts were lost. The book/bible exists, but is never found or spoken of again.

Now assume that you are a newborn human understanding the world around you. You grow up in this supernatural world, one that has a deity, but again, the book and oral history to the deities teachings, as far as all of mankind is concerned, have been forever lost, how do you make the leap of knowledge to accept that there is not only a deity, but that there is the existence of this one exact deity?