God's existence or nonexistence is not contingent on our beilef. That argument would no more work for him than his declaring he believes in God therefore I'm going to hell would work for me. One can not simply declare truth by fiat.
"I contend that we are both atheists. I just believe in one fewer god than you. When you understand why you dismiss all other possible gods, you will understand why I dismiss yours." -Stephen Henry Roberts
http://www.nonestdeus.blogspot.com
http://www.nonestdeus.blogspot.com