Why would people say that atheism is a religion? The face of the question does seem pretty silly and should probably be "Who would say that atheism is a religion?" But people do get told sometimes that, say, atheism or Marxism or secularism is a religion. For instance, when I was a freshman in college, the textbook we used for my Religious Studies class classified those three things as religious viewpoints--as, in fact, religions.
But why? Why would somebody classify them that way? For the man on the street, the classification just seems dumb. What kind of crazy logic does one use to make that statement?
The logic behind these kinds of classifications (atheism, Marxism, secularism are just three more religious instantiations) isn't exactly crazy or convoluted, but it does seem a little silly--think of everything you'd have to include. The enabler of the classification is an expanded definition of "religion." This usually only occurs in academic circles, and one example is religion as "a system of belief." If we are using that definition, then the others all seem to fall into place--sort of. Think of how small an atheist's (as opposed to a Marxist's) "system of belief" is--one belief, "God does not exist." However, if one pulls implications from this, like the truth of, say, existentialism or Darwinian evolution, then one plausibly starts extending the number of beliefs housed under the system of belief called "atheism."
But it's still silly. This all started when the Enlightenment started opposing "beliefs" to "knowledge" (which is actually something that started earlier, with the Greeks). They did so to put Christians on the defensive. You couldn't be an open atheist in the old days, but as the Renaissance wore into the Reformation, and then with the advent of the New Science, atheists (all of whom were intellectuals) became bolder and bolder. And they had a lot of built up resentment over having to be in the closet. The Enlightenment was the explosion of all this. Particularly with Galileo and Newton doing such fine work, they started asserting their supremacy over Christians by making invidious distinctions between tradition and reason, prejudice and rationality, superstition and facts, beliefs and knowledge, etc.
As time has worn on, however, religious intellectuals have learned from their mistakes. In particular, they've gone on the offensive. The Enlightenment was able to make all of these invidious distinctions because they believed (mostly because Kant had said so) that their philosophy was presuppositionless--bereft of assumptions. Christians had made the mistake (from this point of view, a tactical mistake) of resting their philosophy on faith, on the unarguable nature of God. Well, Enlightenment philosophers jumped all over this. The New Science opened up massive hope for not just having to accept things on faith, but being able to prove them. So, everything unprovable, unarguable, must be based on second-rate faith. Like faith that unicorns do, in fact, exist.
Theologians tottered off into a corner, licked their wounds, and began to scrutinize just what had happened. They noticed that these Enlightenment folks liked to talk about everything, that offering clipped "the Bible tells us so" (even when you sing it) isn't good enough for them. They wanted arguments that played by the rules of logic. "Okay," they said, "we'll give you an argument." They honed in on the notion of "presuppositionless," noticing that the atheists' most-used weapons hinged on it. They noticed that for an argument to get off the ground, you need to take for granted certain things--you can't argue about everything all at once. The Enlightenment notion of "presuppositionlessness," however, seemed to suppose that Enlightenment philosophers didn't have to take anything for granted--they had no assumptions. But any argument that is made clearly shows that to be false--every argument has assumptions.
They rolled that around in their heads for a while and eventually figured out that if Enlightenment philosophy was true, it was impossible, but since it was not impossible (it being an historically instantiated actuality), it had assumptions--assumptions that could be attacked, just as their's had been attacked. One line of attack is this: if beliefs are opposed to knowledge based on the fact that you can't argue or prove beliefs and you can about knowledge, then your "knowledge" (for instance, "there is no God") has a background of belief that cannot be argued or proven.
This little story is, of course, not literally what happened (atheists, in fact, had more to do with giving Christians these weapons then they themselves had to do with creating them). But I hope it shows the outlines of how calling atheism a "system of belief" makes sense. This little tall tale is, in fact, what leads directly to the contemporary inflammation of creationism, or intelligent design as they're calling it these days. ID defenders like Phillip Johnson, Ken Ham, and Michael Behe blend together things learned from evil post-modernism with wonderful Bible dogmatism in the weirdest possible way--and yet it is fairly coherent, just really stupid. I take the recent "backlash" against Darwin to be the clearest signal, far more powerful than anything Rorty or anybody else has written, for us to finally and forever ditch Enlightenment philosophy and all of its remenants. Adherence to Enlightenment philosophy and its rhetoric is what allows Johnson and his compatriots a foothold, or a "wedge" as they like to call it. If we ditch Enlightenment, scientistic rhetoric, the wedge has no crack to enter.
To sum up: atheism is a religion only if you define religion in broad, almost useless ways like "system of belief." Such a definition might be useful for certain, narrow academic purposes, but for the most part those of us in the real world need something with a little more bite. If your definition involves the accumulation of multiple labels for a person (i.e., a Christian is also a democratic citizen, meaning that Christianity as a system of belief does not include democracy as a system of belief), that a person is the intersection of a number of "systems of belief", then atheism is a pretty weak system of belief because "system" seems to imply more than one belief: God doesn't exist. Atheism then becomes the call for the abandonment of a particular kind of system of belief. It becomes the suggestion that the sector in our network of systems of belief, where beliefs that revolve around the word "God" exist, should cease to be a sector in which we do any thinking, it should be emptied out and left alone.
So if somebody brings up the "fact" that atheism is a religion for polemical purposes, just counter by saying, "Yeah, okay, if you stretch religion so far as to include atheism, then its a religion. But it's still the religion that says that all this God-talk is pretty pointless." Switching the grounds of debate from "belief" to "stuff we talk about" is fairly effective.