It’s probably better if some Satanist answers this instead of me, but AFAIK there are as many answers for this as there are Satanists out there.

For a lot of them Satanism boils down to a set of moral principles; e.g. embracing individualism, non-conformism and carnal desires as virtues instead of sins. It’s an opposition to Christianity on moral grounds, but it says nothing about agreement/disagreement on epistemic ones. (AFAIK most of those are Atheists.)

For some Satanism is more like an instinct of opposition, internal to the individual, that pops up across multiple religions; e.g. the Set from the Ancient Egyptian religion, the Asurāḥ from Hinduism, and the Satan from Judaism/Christianity/Islamism. And it’s that instinct that they worship/appreciate/support. (I’d argue those are either Pantheists or Panentheists.)

Then for a few it’s like “inverted Christianity” — the epistemic beliefs are the same (there’s some guy called Yahweh creating the world, he create a guy called Satan, Satan backstabs Yahweh), but the morality is flipped (i.e. worshipping Satan instead of Yahweh).

So TL;DR: it depends, but for most of them there’s no belief in the epistemic claims of Christianity.