The Collaborative International Dictionary of English v.0.48:

Unchristian \Un*chris"tian\, a. [Pref. un- not + Christian.]
   1. Not Christian; not converted to the Christian faith;
      infidel.
      [1913 Webster]

   2. Contrary to Christianity; not like or becoming a
      Christian; as, unchristian conduct.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unchristian \Un*chris"tian\, v. t. [1st pref. un- + Christian.]
   To make unchristian. [Obs.] --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unchristian
    adj 1: not of a Christian faith [ant: christian]

Moby Thesaurus II by Grady Ward, 1.0:

82 Moby Thesaurus words for "unchristian":
   Draconian, Tartarean, animal, anthropophagous, atheist, atheistic,
   atrocious, barbaric, barbarous, beastly, bestial, bloodthirsty,
   bloody, bloody-minded, brutal, brutalized, brute, brutish,
   cannibalistic, cruel, cruel-hearted, demoniac, demoniacal,
   devilish, diabolic, disbelieving, faithless, fell, feral,
   ferocious, fiendish, fiendlike, fierce, gentile, godless, goyish,
   heathen, hellish, impious, infernal, infidel, infidelic, inhuman,
   inhumane, irreligious, minimifidian, misbelieving, murderous,
   non-Christian, non-Jewish, non-Mohammedan, non-Mormon, non-Moslem,
   non-Muhammadan, non-Muslim, nullifidian, pagan, ruthless, sadistic,
   sanguinary, sanguineous, satanic, savage, sharkish, sinful,
   slavering, subhuman, truculent, un-Christly, unangelic,
   unbelieving, uncircumcised, uncivilized, unconscionable, ungodly,
   unholy, unhuman, unrighteous, unsaintly, vicious, wicked,
   wolfish