Think, Tap-Dance, God

Descartes's a priori argument that God exists is a version of what's called the Ontological Argument. The Stanford Encyclopedia of Philosophy has an entry on Ontological Arguments. They all have that weird structure:

The idea of God exists in my mind.
[fancy philosophical tap-dancing.]
Therefore, God must really exist.

Most seem to think the problem with the argument occurs somewhere around the tap-dancing.