Corollaries of Atheism codify lexis and praxis in a form very similar to such codifications in other religions. I've linked a couple of these earlier (Humanist Manifestos I and II). If you think about it, there are numerous practical outworkings of the belief there is no God that serve to designate atheism as a religion. A religion needn't have a God with the attributes of invisible, incomprensible, omniponent, omnipresent, etc. to be recognized as such. The God can be man.
But that's not religion, unless I'm mistaken about what the term 'religion' means. Religion and philosophy are not synonymous, although one can be derived from the other. You can have a philosophy - your general outlook on life, i.e., I'm going to do ABC and not XYZ - but it is not, in any understanding I have of the word, a religion.