The Collaborative International Dictionary of English v.0.48:

Apostatize \A*pos"ta*tize\, v. i. [imp. & p. p. Apostatized;
   p. pr. & vb. n. Apostatizing.] [LL. apostatizare.]
   To renounce totally a religious belief once professed; to
   forsake one's church, the faith or principles once held, or
   the party to which one has previously adhered.
   [1913 Webster]

         He apostatized from his old faith in facts, took to
         believing in ?emblances.                 --Carlyle.
   [1913 Webster]

WordNet (r) 3.0 (2006):

apostatize
    v 1: abandon one's beliefs or allegiances [syn: apostatize,
         apostatise, tergiversate]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "apostatize":
   apostacize, betray, bolt, break away, change sides, defect,
   degenerate, desert, fall away, fall off, go over, let down,
   pull out, rat, renegade, renege, renounce, repudiate, run out on,
   secede, sell out, switch, switch over, tergiversate, turn,
   turn against, turn cloak, turn traitor