When someonse's beliefs are the opposite or different from yours, do you call them "retard?"

Only if they call my views retard first. I remember just yesterday I posted a story on one of my Christian friend who justified the violence in Bible. A triggered Christian user first commented I was lying. Then I said I wasn't he proceeded to call me a troll, and whatever names he could think of. He asked many times if I was lying. I told him I wasn't, still he wasn't satisfied and he insulted me more. Then I trolled him for a bit before he finally gave up and left the post.

I try not to respond to religious posts which I do find stupid because I think it's waste of time.