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]