The person is being a smart-alec, but that's okay. Just paste the following...
But it takes an infinite being to have infinite knowledge therefore you destroy your own argument by being the infinite being that you say doesn't exist.
If the person goes on being silly like, "Yes! I'm God", etc., go here.
If the person admits he/she was being silly, paste this...
Okay, so I state again - You would have to have infinite knowledge (know all things) in order to KNOW for a FACT that God does NOT exist. Do you claim to know all things?
If the person asks where's the proof for God/Jesus/Christianity, go here.