I can see why the focus of rebuttal in this thread has been on the first premise, but the one people should be focusing on is the second. It's the one his entire argument is based on, yet he's very cleverly hiding it in the middle as if it's an axiom to draw attention away from it. Look how it reads when ordered this way:
P1. Knowledge is possible.
P2. For knowledge to be possible, God must exist.
C. Therefore, God exists.
See how much more it sticks out now? To get to God existing he must first establish that knowledge is possible, yet he can't do that because the "proof" that knowledge is possible is that God exists. It's utterly circular and therefore nonsensical.
I didnt even notice that. Good catch. His reply here should be pure gold.