God may be defined as the being than which none greater can be concieved.
I have in my mind the possibility of God.
-The God exists in the mind and not in reality.
-But if such a being that exists in the mind and reality is greater than in the mind and not reality.
-That is a contradiction!
Therefore, it is not the case that the being exists in the mind and not reality.
Restated, if the being exists in the mind, then it exists in reality.
God exists in reality.