The good ol presuppositionalist argument. "God must exist because he must by definition."

So must the leprechauns who inhabit my garden. At least the leprechauns bring me slugs each morning! God's just about effective as a gallon of milk on the counter.

Descartes: “The only thing you know for certain exists is yourself because you’re thinking. Perception isn’t necessarily real because you’re just thinking it and could be mistaken.”

Also Descartes: “Clearly god exists because we can think about him”

God's not a figment of your imagination. You're a figment of God's imagination.