Answer
if you saw vma's last night.. it seems pretty probable... he has written songs that exclusivly refer to the devil.. and he has been quoted as saying things like the "devil invented rock and roll" and "if Star Wars didnt have his (the devils) mark of evil it wouldent have sold any tickets"
so... as it seems.. he definetly is.
Answer
While I could not find any articles or interviews that describe what Jack Black's religious/spiritual beliefs are (other than that he was raised Jewish), I seriously doubt he is a Satanist. If you followed his work, you would know that rock 'n roll and comedy are what he's all about...not Satanism.
You can't assume that someone is a Satanist just because they've mentioned the Devil in some of their songs and interviews. Especially since the idea of the Devil is often used symbolically or metaphorically.
I suspect that the above quotations are simply jokes that the first answerer has either misunderstood, or taken completely out of context, or both.
answer
he is because when it was the MTV movie awards . he said bow your heads and pray to the devil.
answer
Jack Black isn't a satanist he's a Jew.
Jack Black is Number One.
Jack Black is AMERICAN
His real name is Thomas J. Black According to Oct 2003 GQ Jack said when he signs a bill at a restaurant, he signs it with his given name, Thomas Black.
Jack is a nickname for Jacob.
It is absolutely Jack Black
No, they're not
No silly he's black
No, Eminem is NOT a satanist.
no he is not a satanist
No, Drake is not a Satanist.
No, Rihanna is not a Satanist.
No Bam Margera is not a satanist, people may think he is because of his taste in music that consists of some Satanist bands but he is not satanist.
Is shabba ranks a satanist
No, Paulo Coelho is not a Satanist.
No, Chris Brown is not a Satanist.
No, Jonathon Ive is not a Satanist.
I think that for sure, he is a satanist, with the things he does.