Though it is clear the song is about questioning belief in a God, I don't think it is meant to be a conversation (between two people or entities). This is someone questioning their own beliefs, an internal monologue. Through internal dialogue realizing the belief in God is individual concept. You could then speculate that if my God let me down, then the God or vision of God you created is faulty (How could you tell me that I'm great).
Though it is clear the song is about questioning belief in a God, I don't think it is meant to be a conversation (between two people or entities). This is someone questioning their own beliefs, an internal monologue. Through internal dialogue realizing the belief in God is individual concept. You could then speculate that if my God let me down, then the God or vision of God you created is faulty (How could you tell me that I'm great).