"We are not debating the existence of God, we are debating if you can be sure of his existence/non existence."
I don't know if this has been brought up before but it would be helpful to define what type of god we are talking about. Is it the Biblical god? Is it a Deist god who only created the universe but left it alone for life to evolve?
You can make absolutely sure that the biblical god either does not exist or if he exists that he is a pathetic writer and tyrant. That would be based on the contradictions and the moral turpitude of the Bible.
The Deist god is harder to "disprove" as he can be considered to be an over glorified scientist in a different universe experimenting with the creation of other universes.