Most or us are no doubt familiar with St Anselm's Ontological Argument for God's existence. For those who are not, it can be found here: http://en.wikipedia.org/wiki/Ontological_argument#Anselm
Having thought about it a bit, I have realised that St Anselm doesn't give God enough credit. He limits God's greatness to "that which can be conceived of". But really, God must be greater than that. So here goes:
1. God is, in principle, such great a being that no greater being can exist
2. A being that is so great that we can't conceive of it is greater than a being that we can conceive of (our minds are limited in their ability to conceive)
3. Therefore God must, in principle, be a being that we can't conceive of
4. But we have conceived of God in 1&3 above
5. Therefore God doesn't exist
On a bright side, not all hope is lost. If we all stop believing (well, those who do) and stop conceiving of God, we might give rise to His existence. Just like Saint Anselm brings Him into the real world by simply imagining Him**. Shall we try?
**Who made that guy a saint? What were they thinking?