It is actually an ironic joke. What constitutes an atheist? Someone that doesn't believe in god. How will most atheists express their views? "I don't believe in god."

In every case, you cannot be an atheist without bringing god into the equation. It's more of a linguistic irony than anything else and I see I have failed to express it correctly using 0 and X. The fact that 0 doesn't represent anything or does not exist is precisely the point, as Atheists believe god does not exist. However, the paradigm of atheism is built on "god" just as much as zero performs an integral duty in mathematics.