Hmmm.
Well they're prog metal aren't they? Elements of Prog rock, Death and Black Metal.
So far as i know there's no real stipulation that says that Prog or Death Metal has any particular affiliation to any God/Jinn/spirit/deity etc or takes any direct opposition against the idea of one.
However certain occult and therefore Satanic, themes and an atmosphere have dogged metal and rock since the days of Led Zeppelin.
Black Metal's a different story isn't it? The very birth of Black Metal from Venoms' album "Black Metal" (1982) was centered around the Satanic theme.
Bands like Mayhem, Emperor and Bathory rather than taking side with Lucifer chose simple to take sides against God and the Judeo Christian religion of Norway.
So sure yes, Opeth name Satan in their lyrics, so what? Would you call them a Satanic band? No.
Best band in the world? FUCK YEA.