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]