The Collaborative International Dictionary of English v.0.48:

Satanist \Sa"tan*ist\, n.
   1. A very wicked person. [R.] --Granger.
      [1913 Webster]

   2. A worshiper of Satan.
      [PJC]

WordNet (r) 3.0 (2006):

Satanist
    n 1: an adherent of Satan or Satanism [syn: Satanist,
         diabolist]