S: (n) religion, faith, religious belief (a strong belief in a supernatural power or powers that control human destiny) "he lost his faith but not his morality"
S: (n) religion, faith, organized religion (an institution to express belief in a divine power) "he was raised in the Baptist religion"; "a member of his own faith contradicted him"
So, going by that definition, it is NOT a religion. It is a belief, but not a religion.
As to those who said that if Satanism and Buddhism are considered religions when they do not worship a deity, then why not Atheism? Well, first of all, I would not consider Satanism a religion. Satanists do not worship Satan or believe in any supernatural powers. As far as I know, Satanism is a philosophy, a set of guidelines for living your life. Not a religion at all.
Buddhism IS a religion because Buddhists believe in supernatural powers. Karma and reincarnation for example. By definition, a religion doesn't need to be centered around a deity, any type of supernatural power will do.
So, revisiting Atheism, Atheists believe that there is no God and (atleast the vast majority) do not believe in supernatural powers of any sort. Which would make me consider Atheism a belief, but not at all a religion. I'm sure that in casual conversation that distinction is bound to be forgotten more often than not, but technically it is not a religion by definition.