Unfortunately your mother is just ignorant, she is somewhat correct about One Winged Angel however, the Japanese culture to this day demonises Christianity, to the point where the Christian god is more likely to be an enemy than an ally in their literature & games.
The point is it's simply a wonderful orchestral piece of music writen in the language of Christianity; Latin.
There are a few ways you can get back at her about this if you feel malicious you can point out that if her spirit is so weak to be tainted by orchestral music she would better service herself by spending the time she wasted, berating you, in church. Seeing as what she's experiencing is the same as homophobia caused by being insecure with your own sexuality.
Or simply listen to Devil May Cry's soundtrack, I reccomend this: http://www.youtube.com/watch?v=cLBAFJWbd30 as it's about killing demons, a very Christian act, going on a holy crusade.
The correct way to go about it would be try to talk to her about it. Your musical taste isn't going to turn you into a pillar of evil, she's lumping everything into two categories, good and evil and she's generalising things she knows nothing about. Ask her to do actual research about aspects of your life (not hers) before attempting to intervene rather than just making knee jerk decisions, if she actually takes that statement to heart she'll almost certainly never bother you again...at least until you get onto the topic of sex.
