“Yet the Bible is a book and therefore must be capitalized. How do you justify capitalizing islam but not Christian?”
I think Christian Bale is capitalized.
If you use Christian as a noun, it is capitalized. If you used Christian as an adjective, you do not (unless it is the first word of the sentence). Basic grammer, I think.
Besides, “that was so islam of you” does not pack as good a punch as “that was so christian of you”. Just my humble opinion.
I think you is right! Islam is always an evil religion (noun).