Fuck 'em. If you're gonna converse with xians, keep the subject to physical things that can actually be demonstrated like historical evidence against xianity, proof of stolen Pagan beliefs and allegories that were incorporated into xianity, anti-Human passages in the bible that promote blood sacrifice, rape and genocide, things of that nature. Even when you attain the ability to astral project if you've not already, there's nothing you can really do to make this person believe you. They'll dismiss you as either a liar or insane, and insist that their beloved lahd is still the good guy and you've just been deceived.
That's not even mentioning the fact that demonstrating any spiritual abilities you develop is a very dangerous thing. You never know how one may react to this, and you risk putting yourself in physical danger, which neither you nor Satan want.
If you find yourself getting angry because of the sheer stupidity of some xian fuck, remember the fate that's waiting for them and the incomprehensibly superior one that's awaiting you. It'll make you happy.