The Collaborative International Dictionary of English v.0.48:

Apostate \A*pos"tate\, a.
   Pertaining to, or characterized by, apostasy; faithless to
   moral allegiance; renegade.
   [1913 Webster]

         So spake the apostate angel.             --Milton.
   [1913 Webster]

         A wretched and apostate state.           --Steele.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Apostate \A*pos"tate\, v. i. [L. apostatare.]
   To apostatize. [Obs.]
   [1913 Webster]

         We are not of them which apostate from Christ. --Bp.
                                                  Hall.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Apostate \A*pos"tate\, n. [L. apostata, Gr. ?, fr. ?. See
   Apostasy.]
   1. One who has forsaken the faith, principles, or party, to
      which he before adhered; esp., one who has forsaken his
      religion for another; a pervert; a renegade.
      [1913 Webster]

   2. (R. C. Ch.) One who, after having received sacred orders,
      renounces his clerical profession.
      [1913 Webster]

WordNet (r) 3.0 (2006):

apostate
    adj 1: not faithful to religion or party or cause
    n 1: a disloyal person who betrays or deserts his cause or
         religion or political party or friend etc. [syn:
         deserter, apostate, renegade, turncoat, recreant,
         ratter]

Moby Thesaurus II by Grady Ward, 1.0:

70 Moby Thesaurus words for "apostate":
   Sabbath-breaker, atheist, atheistic, backslider, backsliding,
   blasphemer, blasphemous, bolter, collaborationist, collaborative,
   collaborator, convert, defector, degenerate, demurrer, deserter,
   disloyal, dissenter, dissentient, dissident, faithless, fallen,
   fallen from grace, fifth columnist, impious, irreligious,
   irreverent, lapsed, mugwump, nonconformist, objector, opinionist,
   opposition voice, profanatory, profane, proselyte, protestant,
   protester, quisling, rat, recidivist, recidivistic, recreant,
   recusant, renegade, renegado, renegate, reversionist, runagate,
   sacrilegious, sacrilegist, schismatic, seceder, secessionist,
   sectarian, sectary, separatist, strikebreaker, tergiversant,
   tergiversating, tergiversator, traitor, traitorous, treasonable,
   treasonous, turnabout, turncoat, turntail, unbeliever, undutiful



The Devil's Dictionary (1881-1906):

APOSTATE, n.  A leech who, having penetrated the shell of a turtle
only to find that the creature has long been dead, deems it expedient
to form a new attachment to a fresh turtle.