There are several god claims made by several religions and all that atheism does is say 'sorry, I don't believe your claims are fundamented'. that's it. there's no positive claim being made, it's just a response to the positive claims of religions.
Ateism doesn't exist unless religious people make claims about supernatural things. So how can you call it a religion? (atheism is as much of a religion as bald is a hair color) ;)
on a side note, agnosticism is the positive claim that 'we don't know if a god exists until it proves itself'. you can be agnostic atheist (i don't know if a god exists, but I believe it doesn't) or agnostic theist (i don't know if a god exists, but I believe it does).
so being agnostic doesn't stop you from being atheist :)
religion as a word is rooted to the 12th century and at the time the meaning was more of a "conciece awareness of the greater influences to which we are tied to". it wasn't until around the 18th century that the word morphed into what you are defining it as today. i prefer the older definition in part because i want to qualify athiest as a religion because i view it as my religion.
the non-religious are the ones that are not conscientious of the "bigger picture". of those that are conscientious, most believe in some sort of a god. some, as i do, don't believe in an external influence such as god and only believe in the world as i see it now. some don't know which belief, if any, is true.
my point is only this: there is a huge distiction between not giving religion any thought and giving religion thought and coming to the conclusion that there is no god.