1) If God exists, he must exist necessarily, if God does not exist his existence is impossible.
(2) Therefore, God is either necessary or impossible.
(3) God can be conceived without contradiction
(4) therefore, God is not impossible
(5) Since God is not impossible he must be necessary.
(6) Since god is necessary he must exist.
The assumption that God cannot be contingent is implicit in the concept of God itself.Therefore God cannot exist contingently.