1. God exists in the understanding but not in reality.

2. Existence in reality is greater than existence in the understanding alone.

3.  God's existence in reality is conceivable.

4. If God did exist in reality, then he would be greater than he is (from (1) and (2)).

5. It is conceivable that there be a being greater than God is ((3) and (4)).

6. It is conceivable that there be a being greater than the being than which nothing greater can be conceived (5), by the definition of 'God').
    

But surely

(7) It is false that it is conceivable that there be a being greater than the

being than which none greater can be conceived.

Since (6) and (7) contradict each other, we may conclude that
(8) If God exists in the understanding, God must also exist in reality.