How to define "Religion" as a whole?

>agnosticism

Atheism, in a broad sense, is the rejection of belief in the existence of deities.[1] In a narrower sense, atheism is specifically the position that there are no deities.[2] Most inclusively, atheism is simply the absence of belief that any deities exist.

Just because you want to make your belief system broader than it is doesnt make it not a belief system.

And in my first post when i'm talking what follows, what actually follows isn't a part of atheism. It's just where the belief invariably takes you.

At it's truly simplest, atheism is the belief that there is no god. the reasoning is irrelevant.

Due to a lack of evidence not a rejection of evidence. It is not (but can be) just saying "There is no God because I say so" it's saying "There is no God because there has yet to be enough substantial evidence that the claim can become plausible".

Agnosticism is the belief that we can't know everything so anything is possible.

@ second part: If anything I narrowed it. I suppose if you REALLY wanted to push it, you could say atheism belief system is that atheist believe there is not enough evidence to support the existence of god, but most atheist don't restrict what is and is not credible to god. As well I suppose you could say what determines somethings credibility is an "trait" of atheism, but that way of thinking is not restricted to atheism

@ second to last part: That's not really 100% accurate. The Piraha are a tribe of what falls under the definition of "atheist" (even by your quotes), but are spiritual in their own way, in that things are not always "just natural".

@last part: It is in determining it's basis for cataloging it as a religion. This is a bit overused, but it's like saying not collecting stamps is a hobby, even if every other person in the world does it, you not doing it does not catalog you within the same category of those that do collect stamps. Then again I suppose it's a chicken and an egg thing in that if religious people claimed god and someone claimed "no" then it doesn't really fall into category, however if the opposite were true you'd be right (although history supports the former).