Atheism is a rejection in the belief of deities. In the real world, that is easy enough to understand. There are many religions practiced on Earth, and most of them (if not all) worship at least one deity. This could be a god or goddess that is associated with a certain characteristic or trait, or it could be a creator, one divine being that shaped everything that is everything from nothing. Whatever your beliefs are, atheism is simply a rejection that any of that happened or exists.
But what happens when we push reality into fiction and bring this thought into the game world that we all log in to? Can atheism really exist in Azeroth? We know that there are gods and goddesses that have roles in WoW. A few of them have even been seen in game, like Hakkar or Yogg-Saron. But there are many that have yet to make an appearance, like Elune or the other Old Gods that we've yet to uncover.