We covered this ontological argument in our Christian Beliefs class. I really enjoyed it. Partly because it really confused me.

"This proposition is indeed so true that its negation is inconceivable. For it is quite conceivable that there is something whose non-existence is inconceivable, and this must be greater than that whose non-existence is conceivable. Wherefore, if that thing than which no greater thing is conceivable can be conceived as nonexistent; then, that very thing than which a greater is inconceivable is not that than which a greater is inconcevable; which is a contradiction.

So true is it that there exists something than which a greater is inconcevable, that its nonexistence is inconceivable: and this thing you are, O Lord our God!

So truly therefore do you exist, O Lord My God, that you non-existence is inconceivable; and with good reason' for if a man's mind could conceive anything better than you, the creature would rise above the Creator and judge him' which is utterly absurd. And in truth whatever else there be beside you may be conceived as non-existent. You alone, therefore must truly of all, and therefore most of all, have existence: because whatever else there is, is not so truly existent, and therefore has less the prerogative of existence. "

[From Proslogion, iii and iv, Eerdmans p. 275] Anselm

So take that.