Lothlorien
Opeth's lyrics are beautifully written stories. Just that, stories. Even if a lyric he writes seems to have to do with being anti-christ or being "evil" doesnt mean it has anything to do with him personally. I am almost positive all of Opeth's lyrics are fictional (correct me if I a am) so why not say that someone who writes a fiction book that has to do with evils and the devil is against christianity?
and
Braighs
Two ways to answer the question:
1. The band denies it. (The fact that they need to is rediculous)
2. The lyrics don't advocate Satanism. (BotH and TGC describe satan, but so does the bible.)