1) This is a Sisphyian task. You have a player who is dead set against a system. No matter what logical appeals you make, he won’t play. He is just as likely rationalizing as he is actually being stopped by whatever argument he presents. I, for instance, detest D&D as a TTRPG. It is unlikely anyone will convince me otherwise, even if they can out-rhetoric or argue a point I make against it. So write him off...he’s not playing and you have a group.

2) As has been mentioned, players who fall into the “obnoxiously unable to separate their real world devotion from fictional characters and settings” are a sub-optimal group to work with period. The only question is how many games they get you to ruin on your own accord for their beliefs before they decide to ruin one themselves.