1. The idea of God is the idea of a greatest possible being (i.e., it is not possible to think of a beign greater than God.)
2. Assume that the idea of God is the idea of a being that exists only as a figment of our imagination and does not exist in reality.
3. If 2 is true, then we can form the idea of a being in our minds that is just like God but that also exists in reality; let's call this the idea of REAL GOD.
4. The idea fo REAL GOD is greater then the idea of God. So, if 2 is true, it is possible to think of a being greater than God.
5. 1 and 4 are contradictory statements
Therefore: Our assumption that God exists only as a figment of our imagination must be false (i.e., God is a being that exists in reality).