To be fair, the Satanic Temple isn't exactly satanic per se. It's more of a parody religion to point out the unfair law exceptions churches get. They are more anti-church (or maybe even anti-religion) than actually satanic.

I'd be very surprised if a significant portion of members of the Satanic Temple actually seriously believe in the existence of satan.

It's about the same as e.g. the Church of the Flying Spaghetti Monster.