'Lord, not only are You that then which nothing greater can be conceived but you are also something greater than can be conceived' St. Anselm, Proslogion.
As Anselm states above, God is that which nothing greater can be conceived, therefore not only must he exist in the mind, but in reality. This is because it is greater to exist in reality than simply in the mind. There is strength in this point, as what Anselm is saying is true. It is greater to exist in reality, that in the mind alone. For example if we had not eaten for a week, it would be good to imagine a 3 course meal, however, to actually receive that meal, and make it a reality, would be even