So God wouldn't exist in this world if nobody believed in him? If nobody believed there was such a thing as oxygen, it doesn't mean oxygen wouldn't exist.
God is an idea, to us, because what we call reality is just a perception of reality. We can't define God, we can only seek Him out. The only way God manifests into our reality is through our belief of Him, this is the way it's set up. So, when we stop believing in Him he ceases to have POWER in OUR world. But he didn't create the world to not have power within it, but he also wants subjects who believe in him. So the moment the last person in humanity stops believing in God is the moment that God holds no power over humanity. The atheist's paradise. But that's not how it ends, not at all.
-Sab