The Collaborative International Dictionary of English v.0.48:

Separatist \Sep`a*ra*tist\, n. [Cf. F. s['e]paratiste.]
   One who withdraws or separates himself; especially, one who
   withdraws from a church to which he has belonged; a seceder
   from an established church; a dissenter; a nonconformist; a
   schismatic; a sectary.
   [1913 Webster]

         Heavy fines on divines who should preach in any meeting
         of separatist .                          --Macaulay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

separatist
    adj 1: having separated or advocating separation from another
           entity or policy or attitude; "a breakaway faction" [syn:
           breakaway, fissiparous, separatist]
    n 1: an advocate of secession or separation from a larger group
         (such as an established church or a national union) [syn:
         separatist, separationist]

Moby Thesaurus II by Grady Ward, 1.0:

52 Moby Thesaurus words for "separatist":
   abjuratory, apostate, backslider, bolter, breakaway,
   collaborationist, collaborator, convert, defector, demurrer,
   deserter, dissenter, dissentient, dissident, fifth columnist,
   misbeliever, mugwump, mugwumpian, mugwumpish, nonconformist,
   objector, opinionist, opposition voice, proselyte, protestant,
   protester, quisling, recidivist, recreant, recusant, renegade,
   renegado, renegate, renunciative, renunciatory, repudiative,
   repudiatory, reversionist, revocatory, runagate, schismatic,
   seceder, secessionist, sectarian, sectary, strikebreaker,
   tergiversant, tergiversator, traitor, turnabout, turncoat,
   turntail