Dictionary.com describes religion as:
1. a set of beliefs concerning the cause, nature, and purpose of the universe, especially 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.Under definition #2 and somewhat in #1 and #5, atheism would qualify as a religion.
At this point, I want to move into my view of what a religion is and how atheism stacks up with those views. Feel free to comment below if you think that I have missed a key element that defines religion in general.
A religion is something where people have shared beliefs. An anti-belief that is held as a belief is still a belief no matter how much one wants to pretend that an anti-belief is not a belief. Shared beliefs among atheists are that there are no gods, science can explain everything eventually, and reason is king. Please correct me if I am wrong and have misrepresented atheism.
Having shared ethics is not essential to being a religion. If ethics were necessary to be a religion, then that would eliminate Christianity. Once you zoom in and have a closer look at all the diverse expressions of Christianity (Reformed, Baptist, Methodist, etc.), you would see shared ethics, but Christianity is still a religion without the shared ethics that comes only from zooming in on particular subsets of the Christian religion.
Religions typically proselytize. Richard Dawkins spends nearly a half hour in Richard Dawkins On Militant Atheism encouraging atheists to be more evangelistic about their lack of belief. Atheists do this more aggressively than any group except Christianity in my experience. It is understandable for a person who believes their view is the best for humanity to then share that view with the rest humanity. Actually, I would not respect the person who did otherwise.
A bad religion demonizes those that don't agree with them. Unfortunately, many atheists also frequently fall into this category. They label believers in gods or God as idiots and unintelligent, frequently mocking them. You can see this in I Hate Religion, And Jesus Too, a rebuttal to the recent internet phenom poem Why I Hate Religion, But Love Jesus.
Obviously, "religion" has a negative connotation in atheist circles. But if we don't categorize atheism as a "religion", what should we categorize it as? A world view? A belief system? An anti-belief system? It gets difficult because atheism is usually contrasted with Christianity, Islam, Judaism, Hinduism, or Buddhism. They all compete in the same societal space for the mind of humanity. What is the broad category that includes all of these competing ideas for the mind?
By saying atheism is not a religion, I think atheism runs into the arrogance that I see Christianity run into when they try to claim that Christianity is not a religion. "It's not a religion because it's right" is the arrogant mentality that some Christians have. Others proclaim, "It's a relationship, not a religion." "Religions are [this] and [that], but we are not [this] and [that]."
Back to the leprechauns. If 60% of the population believed in leprechauns, lived their life according to the teachings of the imaginary leprechauns as explained by their prophets, and the other 40% didn't, I would think we would have two different religions. More than likely, the other 40% might fragment into other groups, and we would have more than just two religions. The goblin followers, the followers of no one but the thoughts of man, and the followers of the moon. What is the overarching category to contain all 100% of human beliefs, even if they are crazy? Is not "religion" a good word for this?
Then again, maybe not. The big question is what broad category contains all of those competing ways of thinking about God or not about god. For most, "religion" is the word used. It's only derogatory to atheists because they don't like religion. But don't feel bad, my atheist friends. You are not alone. Christians who don't want to be called a religion feel the same way.
A few useful links (all from an atheistic perspective) that I found in the follow-up discussions to this post:
Different Types of Atheism by Martin Willett
Belief Vs Disbelief by Austin Cline
Philosophy Vs Religion by Susan Quilty
You might also be interested in a few other posts.
A Response to Claims of Atheistic Persecution
Tax The Churches