You've got to sort of believe it or at least suspend your disbelief while you are actually singing it, I'd say.

It isn't just religious songs this applies to, it goes for political songs well, and probably a few more as well. (Songs about people taking vengeance, for example.)

I'd also say there has to be some sympathy between what you feel yourself and what you sing. I could sing a song coming from a religious tradition I don't accept, but not one that preached hate. I could sing a song advocating violence in the cause of justice, though I'm a pacifist, but not one that supported repression.