Renee Descartes, after proving the existence of God to his satisfaction, uses God to get himself out of the radically skeptical corner he painted himself into with his cogito. He reasons that God is perfect, and a perfect Being would not deceive him, as deception is a form of imperfection. Therefore he can deduce that the world he experiences is real and not a product of God deceiving his senses.
I think that the same line of reasoning can be used to prove that God does not exist:
1. God is perfect.
2. Deception is a form of imperfection, and a perfect Being would not deceive us.
3. Therefore a perfect being would not deceive us by creating us in such a way that we would perceive things that are not true.
3a. AND a perfect Being would not lie to us.
4. What we perceive often contradicts what God told us.
C. God doesn't exist. QED
I really enjoy this argument. There is something satisfying about turning the Ontological Argument on its head and defining God out of existence. And it reminds me of the argument from Hitchhiker's Guide To the Galaxy that the Babel Fish is so obviously a proof for God's existence that it proves He doesn't exist. Unfortunately, I don't think the argument is sound.
Premise one may not be true, and the argument only disproves the existence of a perfect God. Premise two may not be true, and perfection may include the ability to do all things, including to deceive. Premise five may not be true, and is in fact the subject of a lot of apologetics that attempt to reconcile religion and science. I'm afraid that the whole thing feels like sophistry. Still fun though.