Athiesm is not a religion.

"Religion- A set of beliefs concerning the cause, nature, and purpose of the universe, especially when considered as the creation of a superhuman agency or agencies, usually involving devotional and ritual observances, and often containing a moral code governing the conduct of human affairs."

To simply say there is no God does not come with a substitute belief about what did create the universe. Many athiests happens to share one, but athiesm as a concept does not have anything to do with it. It does not have a set of beliefs about what the nature of the universe is, simply rejection of one idea. Athiesm does not indicate purpose of any kind. Athiesm does not involve any devotional or ritual observances, a moral code, or anything else that defines something as a religion.

To call athiesm a religion is generally a sad attempt at a straw man fallacy, to give the religious some claim that was never made to attempt to refute (such as holes in the theory of evolution or the big bang theory), usually as a diversion to shift the burden of proof away from themselves. Athiesm is a religion in the same sense that not colelcting stamps is a hobby, not smoking is a habbit, transparent is a color, and bald is a hair style.

I actually find it really funny that christians attack athiests for not having a perfect understanding of their faith before attacking it, meanwhile you've decided what all athiests believe without knowing a damn thing about them. To be athiest does not imply acceptance of the big bang theory, evolution or darwinism, or anything else. Simply the rejection of the divine. That is the only belief all ahtiests must share to be athiests, and that doesn't fit any one of the parts of the definition of religion.