1. God exists in the understanding
2. Existence in reality is greater than existence in the understanding alone.
3. A being having all of God’s properties plus existence in reality can be conceived.
4. A being having all of God’s properties plus existence in reality would be greater than God.
5. Such a greater being cannot be conceived, by definition, God being the greatest possible being conceivable.
6. Hence it is false that God exists in the understanding but not in reality.
7. Therefore, since God exists in the understanding, he exists in reality as well.