How to define "Religion" as a whole?

Atheism is arguably a religion or belief system what follows are explanations that are non divine

Atheism does not deny the possibility of divine, simple that there is not enough evidence to support the claim of the divine.

As well it has nothing to do with the explanation of the natural world. It is simply

Religious person: There is God.
Atheist: Prove it.
Religious Person: I can't.
Atheist: Then I don't believe you.