Where this notion derives from, I haven't the faintest clue. It would seem that some religious folks and atheists hate if there isn't a broader distinction between themselves, but under every outlet, such a belief defeats itself.
In this article, I will provide you with all that is necessary to debunk this little matter. I am aware that a quick google search will turn up alternative ideas. If you'd like me to address them, just post the article and I'll get to it whenever I can.
The obvious argument is "Atheism is not a Religion. It's the absence of Religion."
This is a very popular misconception as people tend to confuse theism with religion. The two are different concepts and are not mutually exclusive. But to serve a better model, I'm going to present the Dictionary definitions of the words Religion, Atheism, Atheist and Theism.
a person who denies or disbelieves the existence of a supreme being or beings.
1. the doctrine or belief that there is no God.
2. disbelief in the existence of a supreme being or beings.
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.
1. the belief in one God as the creator and ruler of the universe, without rejection of revelation (distinguished from deism ).
2. belief in the existence of a god or gods (opposed to atheism ).
By the very definitions of the words, we can begin to get a grasp of what these words actually mean.
• Religion being a any view, spiritual or otherwise, regarding life, in of itself.
• Atheism being a belief that there is no God.
• Theism being a belief that there is a God or multiple Gods. (See: Polytheism and monotheism for more.)
Being that atheism is, indeed, a belief in something -- id est, "there is no supernatural deity" - it's a self-defeatist point to make that it is not a religion. Such a statement would imply that Buddhism is, also, not a Religion (no, they don't think Buddha is a deity.)
It can be argued that agnosticism (the middle view; unsure of the existence of a supernatural deity) is not a Religion, but only on a case by case basis. In most circumstances, that, in itself, is also a religious view. The reason I say most is because some people are just so ignorant to such things that it's not that they've reviewed both sides and made a decision, but rather, that they don't have enough information to make a distinction.
But not all atheists believe in the Big Bang Theory!
That's irrelevant. All atheists believe that the origin of the universe is, most definitely, not the result of a supernatural deity. Which would still fall under the aforementioned category.