That's the question asked by Tom Stoneham in his blog:
A philistine [someone who thinks that there is no such thing as aesthetic value] thinks that philosophical aesthetics has no subject matter, while an atheist thinks that philosophy of religion has no subject matter. What is the difference?
I was going to respond to this by email, but the issue is interesting enough that I thought I would write about it here. The answer, though, initially at least, is pretty straightforward and can be found on this very site where you can find a growing amount of material on the philosophy, psychology, and sociology of religion. And why can an atheist write about those topics? Because atheists don’t believe that they have “no subject matter.”
Those fields quite obviously have a subject matter: religion. Or, more specifically, they have the subject matter of religious history, religious beliefs, religious behavior, religious ritual, and quite a lot more. A person doesn’t have to believe in a god in order to study theistic religions or to study the arguments used for and against the existence of gods. Even those who do believe in a god may not always believe in the gods the arguments being examined purport to prove.
A person doesn’t need to be an existentialist in order to teach about and study existentialism. A person doesn’t have to be a liberal in order to teach about the history and philosophy of liberalism. By the same token, a person doesn’t have to be a theist in order to teach about and study the philosophy of religion. The philosophy of religion is not an exercise in theology; as long as there are religious people, the philosophy of religion will have a subject matter to study and teach.
Now, one might question whether an atheist can be fair in teaching the philosophy of religion — after all, an atheist doesn’t believe in any gods, so how can she fairly represent belief in and arguments for them? This same question can be directed at theists: how can a theist fairly represent argument against gods and arguments for gods other than the one(s) he believes in? Theists are in no superior or more secure position when it comes to this topic than nonbelievers.