The Collaborative International Dictionary of English v.0.48:

Apologist \A*pol"o*gist\, n. [Cf. F. apologiste.]
   One who makes an apology; one who speaks or writes in defense
   of a faith, a cause, or an institution; especially, one who
   argues in defense of Christianity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

apologist
    n 1: a person who argues to defend or justify some policy or
         institution; "an apologist for capital punishment" [syn:
         apologist, vindicator, justifier]

Moby Thesaurus II by Grady Ward, 1.0:

68 Moby Thesaurus words for "apologist":
   Maecenas, Philadelphia lawyer, abettor, admirer, advocate,
   aficionado, angel, apologete, apologizer, arguer, argufier, backer,
   buff, casuist, champion, controversialist, debater, defender,
   dependence, disceptator, disputant, encourager, endorser, exponent,
   fan, favorer, friend at court, guard, guardhouse lawyer, justifier,
   logomacher, logomachist, lover, mainstay, maintainer, mooter,
   paladin, paranymph, partisan, patron, pilpulist, pleader, polemic,
   polemicist, polemist, promoter, proponent, protagonist, protector,
   reliance, second, seconder, sectary, sider, sponsor, stalwart,
   standby, successful advocate, support, supporter, sustainer,
   sympathizer, upholder, vindicator, votary, well-wisher,
   whitewasher, wrangler