I can certainly see why they would dismiss the genre as overtly satanic, as many of these bands do in fact dabble in this crap or at least lead hedonistic lives. But whether that has to do with the style of music itself, or merely the expression, is debatable.
The lyrics for "Samarithan" by Candlemass are overtly about the virtue of charity (I would debate Christian charity) while the style itself is a slow and lumbering Metal.
Yes, I do believe many times the expression is clearly Satanic... all black metal, much of death metal, and even some more mainstream 'hard rock'... KISS' costumes, for example, and Simmons' tongue clearly seems to be TRYING to represent Satan. Ozzy's title, Prince of Darkness... though some of this might be merely 'marketing', there's still probably something behind it. But I don't believe the style, in and of itself, is necessarily Satanic.
Those lyrics are beautiful.
There are some songs I've found whose lyrics seem explicitly almost Catholic... they both reference more the Crusades, but the choruses... they're both much faster, more upbeat power metal, however.
And then there are some songs that are just literally history... much of Sabaton and also some like this...
I just don't understand how people can dismiss the entire genre as completely Satanic, based on the style of music alone...