Kerry King, the guitarist of Slayer, is opposed to all religion. Tom Araya, the base player/singer, is a practicing Catholic. The only real "satanists" in metal are usually the sort of people who are in the Norwegian Black Metal bands; now those people are absolutely crazy, even the "for show" ones. They make Manson's Anti-Christ Superstar look like gospel music.
Back on Slayer, I like the old music like Reign in Blood or Seasons of the Abyss but the new stuff isn't very good at all. Every time King gets an anti-religious thought he writes a damn song about it and its gotten annoying and repetitive, even hardcore Slayer fans are finally starting to give it up.