Ryujin
Legend
To not believe in something is the opposite of believing in something, or the opposite of faith if you will. In our world it's called atheism, in a D&D world it might be called wilful ignorance depending on how active the gods actually are.
I prefer to call it "belief in nothing." No gods. No afterlife. It's a world view, no less than is theism. In this way it can be used as a "faith" in D&D.
Ever hear of Secular Humanism?