[OMFG!You'reActuallySmart!]

Those who call atheism a religion by definition are adhering to a different definition of religion. When you look at it, generally religion involves a belief in some being. Atheism for obvious reasons lacks this. However, this argument is a bit iffy because some definitions of religion don't specify a being. The stronger argument is that religion, whether it involves a divine being or not, consists of a shared set of beliefs that are practiced. There is no set of beliefs for atheism, it's simply the belief in no god. Plain and simple. Many do adhere to science but not all as it's not a requirement for the definition to be met. Some atheists adhere to a philosophy for their life but no divine being. Once again, a specific philosophy unique to others isn't required.

Thus, if atheism is a religion, then the definition of religion that is being used is not the traditional one, it's a different, generally unspecified one. If atheists fall under the unspecified definition of religion, then agnostics do also because they do not have a set of beliefs, they're more in the middle. However, like atheism they have only one main belief and if atheism is deemed a religion for that very reason, then agnosticism must also be. If agnosticism is not deemed a religion under this unusual definition of religion, then atheism must not be.

What the definition that is used by those who claim this to be true I have no clue.