"Religion" does not mean "to have a belief." If that would be true, then having the belief that the healthcare system should be tax payer funded would be a religion. Having the belief that walking instead of cycling is a better way to getting to places would be a religion. Having the belief that you should cut your grass twice per month would be a religion.

Is atheism a philosophy? Sure, technically, though how intentional it is will vary from person to person. Is it a religion? No, and that's a patently absurd thing to say. Atheism is, in effect, a lack of religious beliefs. You're saying that a lack of religious beliefs is in itself a religious belief. The furthest you could really go is to say it's a perspective on religion, but a religion itself it is not.