By far the greatest proof against the existence of God (their God at least) is the calibre and personality of the people he is supposed to support.
What deity in his right mind would cast his personal aura around such a motley bunch of smug, self-centred, hypocrites? Their only claim to such signal honour is the cosmic breadth of their own egos!
Furthermore, for a God who is supposed to be compassionate, his incredible meanness is ample proof that such a God cannot exist. Some of us may live our lives earnestly seeking the truth and wanting to live honestly. Consequently, we will not believe in a God. What is more, we only do this because of our upbringing - we are innocent! God, in his almighty wisdom, judges us to be evil, and despite our suffering in this life (for wanting to live honestly) we are condemned to eternal punishment in the flames of hell!