The Collaborative International Dictionary of English v.0.48:

Apostasy \A*pos"ta*sy\, n.; pl. Apostasies. [OE. apostasie, F.
   apostasie, L. apostasia, fr. Gr. ? a standing off from, a
   defection, fr. ? to stand off, revolt; ? from + ? to stand.
   See Off and Stand.]
   An abandonment of what one has voluntarily professed; a total
   desertion of departure from one's faith, principles, or
   party; esp., the renunciation of a religious faith; as,
   Julian's apostasy from Christianity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

apostasy
    n 1: the state of having rejected your religious beliefs or your
         political party or a cause (often in favor of opposing
         beliefs or causes) [syn: apostasy, renunciation,
         defection]
    2: the act of abandoning a party for cause [syn: apostasy,
       tergiversation]

Moby Thesaurus II by Grady Ward, 1.0:

128 Moby Thesaurus words for "apostasy":
   Evangelicalism, Protestantism, Reform, Zwinglianism, about-face,
   accommodation, adaptation, adjustment, agreement to disagree,
   alienation, alteration, amelioration, atheism, backsliding,
   betrayal, betterment, bolt, break, breakaway, change,
   change of heart, changeableness, constructive change, continuity,
   conversion, counter-culture, crossing-over, defection,
   degeneration, degenerative change, dereliction, deserter,
   desertion, deterioration, deviation, difference, disaccord,
   disagreement, disapprobation, disapproval, discontinuity,
   disloyalty, disparity, dissatisfaction, dissension, dissent,
   dissentience, dissidence, divergence, diversification, diversion,
   diversity, dropping out, faithlessness, fall from grace, falseness,
   fitting, flip-flop, going over, gradual change, impiety,
   impiousness, improvement, irreligion, irreverence, lapse,
   lapse from grace, melioration, minority opinion, mitigation,
   modification, modulation, new theology, nonagreement, nonassent,
   nonconcurrence, nonconformity, nonconsent, opposition, overthrow,
   perfidy, qualification, radical change, ratting, re-creation,
   realignment, recidivation, recidivism, recreancy, recusance,
   recusancy, redesign, reform, reformation, rejection, remaking,
   renewal, renunciation, repudiation, reshaping, restructuring,
   reversal, revival, revivification, revolution, schism, secession,
   shift, sudden change, switch, tergiversation, total change,
   transition, treacherousness, treason, turn, turnabout,
   turning traitor, underground, undutifulness, upheaval, variance,
   variation, variety, violent change, walkout, withdrawal,
   worsening