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]