When you consider the fact that religion does not require a belief in a divine or supreme being then atheism can be a religion.
Religion | Define Religion at Dictionary.com
a set of beliefs concerning the cause, nature, and purpose of the universe, esp. 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.
2.
a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects: the Christian religion; the Buddhist religion.
3.
the body of persons adhering to a particular set of beliefs and practices: a world council of religions.
4.
the life or state of a monk, nun, etc.: to enter religion.
5.
the practice of religious beliefs; ritual observance of faith.
6.
something one believes in and follows devotedly; a point or matter of ethics or conscience: to make a religion of fighting prejudice.
7.
religions, Archaic . religious rites.
8.
Archaic . strict faithfulness; devotion: a religion to one's vow.