Something is greater if it exists than if it doesn't.If God is the greatest thing imaginable, he must exist. I may paint an imaginary masterpiece, but that only means i imagine that i paint a masterpiece, in fact, since it does not exist, it is no better than my actually existing 'inferior'paintings. A real masterpiece must always be better than an imaginary one!

Note from Joe Offer: this appears to be a quote from St. Anselm of Canterbury (1033-1109) - see here (click)