On a message board today I ran across a girl who asked how atheists respond to the assertion that atheism is a religion. There were a few responses before mine, offering the definition of religion as proof that atheism can't be classified as a religion and offering those golden little one liners that Dawkins and others have made famous - atheism is a religion in the same way not collecting stamps is a hobby - and so forth. I am less interested in trying to convince people that atheism isn't a religion and more intrigued by why anyone would claim such a thing in the first place. Here is my anecdotal experience with talking to a theist about atheism being a religion and the conclusions I drew from it:
I used to work with a really nice Mormon kid who felt that atheism was a religion and the ensuing conversation we had was pretty interesting. Basically I feel all you can do is ask the person what their definition of 'religion' is and then go from there. My friend, for instance, claimed that any belief pertaining to existence was a religion. I asked him if he felt evolutionary biology was a religion and he said said no, but the fact that atheists believe that there isn't a god when it's impossible to prove specifically that god DOESN'T exist makes it a religion and not in the same category as a scientific study. So, he was already moving the goalposts, so to speak, on his definition. I pointed this out to him and he agreed that he had changed his definition but maintained that his position remained firm.
Then I brought up the legality of what a religion is. I pointed out that it wasn't possible for atheist organizations to attain the same tax status as religious organizations (at least not in Washington state as far as I'm aware) and he said he thought that wasn't fair but he still maintained his position. Finally, I asked him why he would consider atheism a religion, given that atheists can't attain the same tax exempt status as religious organizations, that the definition of religion seems to exclude atheism, and that even his atheism-inclusive definition of what a religion was seemed to fall apart instantly under a slight amount of scrutiny. His response was that he didn't like how critical atheists are of religion when, in his opinion, atheists have no more earthly validity to their position than anyone else.
This is the part of the conversation that I found really interesting.
Like I had mentioned, my friend is a Mormon. I asked him how he felt when people would call Mormonism a cult and he rolled his eyes and said that that was ridiculous. I said that I felt that people who classified his religion as a cult were doing the exact same thing with Mormonism that he was doing with atheism. Calling Mormonism a cult is a way to underhandedly insult the religion by expanding the definition of an unsavory term to include the religion. Calling atheism a religion is the exact same practice in that atheists have the same negative reaction to being called religious as Mormons have being called cult members.
My point is this - if someone claims that atheism is a religion I feel like it's more of an expression of their insecurity and lack of understanding of atheism rather than an actual position. Whether you want to spend any of your time dealing with it is your choice, but it's an easily refuted assertion if dealing with the established definitions of atheism and religion. If the conversation goes beyond that, I would probably assume the person is being passive aggressively insulting.