The Collaborative International Dictionary of English v.0.48:

Heretical \He*ret"i*cal\, a.
   Containing heresy; of the nature of, or characterized by,
   heresy.
   [1913 Webster]

WordNet (r) 3.0 (2006):

heretical
    adj 1: characterized by departure from accepted beliefs or
           standards [syn: dissident, heretical, heterodox]

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "heretical":
   Albigensian, Arian, Bohemian, Catharist, Donatist, Ebionitist,
   Erastian, Gnostic, Jansenist, Jansenistic, Jovinianist,
   Jovinianistic, Lollard, Manichaean, Monophysite, Monophysitic,
   Montanist, Montanistic, Pelagian, Sabellian, Waldensian, Wyclifite,
   aberrant, abroad, adrift, agnostic, all abroad, all off, all wrong,
   amiss, antinomian, apocryphal, apostate, apostatical, askew,
   astray, at fault, atheistic, awry, beat, beside the mark,
   breakaway, corrupt, creedless, deceptive, defective, delusive,
   deviant, deviational, deviative, differing, disagreeing,
   disbelieving, dissentient, dissenting, dissident, distorted,
   emanationist, errant, erring, erroneous, faithless, fallacious,
   false, far out, faultful, faulty, flawed, free and easy,
   freethinking, fringy, godless, heathen, heretic, heterodox, hippie,
   hylotheist, hylotheistic, iconoclastic, idolatrous, illogical,
   illusory, impious, incredulous, individualistic, infidel, informal,
   inner-directed, irreligious, kinky, maverick, minimifidian,
   misbelieving, miscreant, nonbelieving, nonconformist, nonorthodox,
   not cricket, not done, not kosher, not right, not true,
   nullifidian, off, off the track, offbeat, original, out, pagan,
   pantheist, pantheistic, peccant, perverse, perverted, repudiative,
   revisionist, sceptic, schismatic, sectarian, self-contradictory,
   straying, unaccepted, unapproved, unauthentic, unauthoritative,
   unbelieving, uncanonical, unconfident, unconventional, unconverted,
   unconvinced, unfactual, unfashionable, unorthodox, unproved,
   unscriptural, unsound, untrue, way out, wide, without faith,
   wrong