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.
Atheism is the lack of belief in God, so this definition does not match.
2.
a specific fundamental set of beliefs and practices generally agreed upon by a number of persons or sects
Specific fundamental set of beliefs, that isn't atheism. Atheism is merely the lack of belief in God, so this definition also does not work.
3.
the body of persons adhering to a particular set of beliefs and practices: a world council of religions.
Pretty much the same as 2.
4.
the life or state of a monk, nun, etc.: to enter religion.
Nope.
5.
the practice of religious beliefs; ritual observance of faith.
The lack of belief in God is not religious, so this also does not work.
6.
something one believes in and follows devotedly; a point or matter of ethics or conscience: to make a religion of fighting prejudice.
This one is difficult to explain.
This definition of religion is specific unto itself. Whereas Christianity can be a religion under most of the other definitions, it can't be a religion under this definition. This definition of religion is purely used to describe
devotion to a belief, which is different than actually
being a belief. Because of this, Atheism is not a religion, but one can become an atheist and act religiously about it (as much as someone can act religiously to legalize marijuana).
In short, this isn't atheism.
7.
religions, Archaic . religious rites.
8.
Archaic . strict faithfulness; devotion: a religion to one's vow.
And this does not describe atheism.
After going over this, I have concluded that atheism is not a religion.
Here's the kicker. Theism isn't even a religion!Theism simply describs the belief in a higher being. Theism does not have a set of beliefs, it
IS a belief. Christianity, on the other hand, is not a belief, it
is a set of beliefs.