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.
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         A wretched and apostate state.           --Steele.
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The Collaborative International Dictionary of English v.0.48:

Apostate \A*pos"tate\, v. i. [L. apostatare.]
   To apostatize. [Obs.]
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         We are not of them which apostate from Christ. --Bp.
                                                  Hall.
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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.
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   2. (R. C. Ch.) One who, after having received sacred orders,
      renounces his clerical profession.
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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]

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.