Rude to Christians? They're the ones who are saying their religion tells them that they can't call other people what those people want to be called. That is the rude thing. How would you like it if I used a pronoun for you that was not the one you went by? You'd probably think it was rude. How about if I said it was against my religion to call you by the pronoun that fits your gender? You'd probably still think it was rude.

And it would not be rude to tell them off for doing so.

That said, in my experience, most trans and nonbinary people are incredibly patient with people like you who refuse to use their pronouns even though you're being extremely rude to them.