diabolist vs devil worshiper

diabolist and devil worshiper both are nouns.

Word	Noun	Adjective	Verb	Adverb
diabolist	Yes	No	No	No
devil worshiper	Yes	No	No	No

As nouns, devil worshiper is a hypernym of diabolist; that is, devil worshiper is a word with a broader meaning than diabolist:

Other hypernyms of diabolist include adherent, disciple.

diabolist (noun)	devil worshiper (noun)
an adherent of Satan or Satanism	someone who worships devils