Pyronide

Personally, I wouldn't go so far to say that Atheism isn't a 'religion'.
A religion would be based off of a set of higher beliefs conservatively kept, mainly pertaining to a divine entity, but alternatively can be something material.

What makes individuals think otherwise is the fact that it is just one that doesn't promote 'spirituality' and 'faith' in a divine entity. (Science not being a divine entity. It is a matter of study and a philosophy that we can materialistically comprehend.)

...

Though, doesn't make you think that if perhaps Atheism can be considered a religion, then couldn't any philosophy for that matter be considered a religion. Therotetically, yes, it could. However, as far as majority comprehension, it just isn't.

-Pyronide

re‧li‧gion  /rɪˈlɪdʒən/[ri-lij-uhn] –noun

1. 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.