One Does Not Simply

I don't get what you're trying to say.

Since there is neither agreement on a clear definition what people mean be the term "god", nor is there presently any mathematical-physical proof if such (whatever) god exists or doesn’t exist, and furthermore "believing" means "not knowing", the statement "I don't think (a) god exists" is clearly a belief.

But Atheism isn't a religion, not in the slightest.