Even as a Christian, it bugs me when people do stuff like that. Religion and art are two separate things unless its art that's specifically about religion. Don't let these people bug you too much, you're art is beautiful no matter the influence or what people say.
It's one of those things that religion needs to stay out of, like goverment. If it is a religious piece, ie, you were drawing Jesus or Satan or Jehovah or even Buddha, for a religious purpose, I can some what seeing people being "you know.. I don't believe in your religion but your piece of work is still pretty fucking amazing." but to just base something that doesn't have to do with religion, on your own religion doesn't make any sense to me... If you were making a piece bashing Catholics or Satan Worshippers Or Wiccans, I can see getting pissed, same with it was with a race/gender/sexuality but..... I don't know.. Why are crosses and skulls and horns automatically assumed to be religion... All the girls at my school with the cheetah print cross shirt or the girls with the skull leggings aren't considered overly religious...
I don't get people at all... ._. I think I'ma just stay being anti social.
At that point you know you can't reason with that kind of drop dead stupid. I'd just play along with it. "I'm going down to the abortion store now for my daily fix and on the way home I was thinking of picking up a gay to sacrifice to the satan lord. Need anything while I'm down there?"