Atheism does not simply reject the claim that there is an old white guy with a white beard and flowing robe who lives in the clouds.
I know few Christians who believe in that cartoon version of religion.
Atheism overtly rejects the entire theistic notion that there is a purposeful creative force behind the design of the universe, and a spiritual realm outside the boundaries of our physical perception.
Nietzsche summed up atheism quite well when he rejected the religious possibility of a spiritual realm, and that the only meaning to life was to strive for creativity and imagination in this physical world.