1) God is the greatest conceivable Being/Thing.
2) In order for God to be the greatest conceivable Being/Thing He must exist not only as a concept but also in reality. Otherwise, something greater can be thought of (a being/thing that exists in concept and reality).
3) Nothing Greater can be thought of than God (A being/thing that is all knowing, all powerful, and present in all places).
Conclusion: God exists in reality