Is atheism a religion?
Is atheism a religion? is a question atheists are often asked, although it's sometimes not asked, but rather stated, "atheism is a religion." I understand the confusion, and there are several points of commonality between the two. Many atheists "religiously" hold meetings where they have "communion" with each other and discuss belief in gods, souls, and the supernatural and "evangelize" their position and try to "convert" people over to it. To many people, this behavior is very similar to the behavior of religious people, so they conclude that atheism is a religion. However, while there are surface similarities, atheism is dramatically different from a religion.
Religions are opt-in
Excluding the more violent believers, most religious people accept that a person must actively chose to follow a religion in order to be religious. Atheism, at least in its implicit and weak forms, commonly described as a "lack of belief," is the default position regarding belief in gods. To put it more pithy, atheism is a religion like not collecting stamps is a hobby.
Generalizing religion weakens religion
If you generalize the definition of a religion so much that you include anything where people gather to talk about a shared interest, it only weakens the idea of religion. For example, people who meet to talk about fantasy football leagues could be seen as "religious," scrap-booking becomes a "religion," and so on.
Religions believe in the supernatural and have rituals
While all religions are different, they do share two common traits, and that is a belief in some form of supernatural phenomena (gods, souls, spirits, etc.) and an adherence to ritual (prayers, rites, tenants, etc.). Atheism is merely the belief that gods do not exist; that's it.
Some religious people are atheists
To add to the confusion, some religions, like certain forms of Buddhism, do not believe in gods. Therefore, those Buddhists are technically atheists even though they believe in souls, pray, and religiously perform rituals. Of course, religious people rarely identify as an atheist, because they equate atheism with non-religious even when it technically isn't.
You can debate a belief without believing in it
This should go without saying, but a common argument in favor of atheism being a religion is that atheists often talk about belief in gods. Of course, just like Conservative pundits can criticize liberal politics without being a Liberal, so too can an atheist criticize theism without being a theist.