My 17 yr old son joined a devil worshipping cult. Should I try to forbid it, or will that just encourage him?

I've already had a conversation with him about it, and he's absolutely adament about staying a member. I should've seen the writing on the walls that time I caught him watching "Gilmore Girls". Now he's always muttering things under his breath like "give me power, O great Lucifer". It's gotten to the point where I'm terrified of him.