How to define "Religion" as a whole?

There's many different types of atheisms, http://en.wikipedia.org/wiki/Atheism
Each one has different ways to go about not believing in god. They are not at all the same and have very different ways in them.

@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).

You are claiming that all religions believe in god, when none of the definitions until now have claimed a religion must have a god/deity. Not even your dictionary definition says it.