The Collaborative International Dictionary of English v.0.48:

Faithless \Faith"less\, a.
   1. Not believing; not giving credit.
      [1913 Webster]

            Be not faithless, but believing.      --John xx. 27.
      [1913 Webster]

   2. Not believing on God or religion; specifically, not
      believing in the Christian religion. --Shak.
      [1913 Webster]

   3. Not observant of promises or covenants.
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   4. Not true to allegiance, duty, or vows; perfidious;
      trecherous; disloyal; not of true fidelity; inconstant, as
      a husband or a wife.
      [1913 Webster]

            A most unnatural and faithless service. --Shak.
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   5. Serving to disappoint or deceive; delusive; unsatisfying.
      "Yonder faithless phantom." --Goldsmith. --
      Faith"less*ly, adv.Faith"less*ness, n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

faithless
    adj 1: having the character of, or characteristic of, a traitor;
           "the faithless Benedict Arnold"; "a lying traitorous
           insurrectionist" [syn: faithless, traitorous,
           unfaithful, treasonable, treasonous]

Moby Thesaurus II by Grady Ward, 1.0:

81 Moby Thesaurus words for "faithless":
   Machiavellian, agnostic, ambidextrous, apostate, artful, atheist,
   atheistic, capricious, changeable, changeful, collaborative,
   conscienceless, crafty, creedless, crooked, cunning, deceitful,
   derelict, disaffected, disbelieving, dishonest, disloyal, double,
   double-dealing, double-faced, double-minded, double-tongued,
   doublehearted, doubting, duplicitous, false, false-principled,
   falsehearted, fickle, fluctuating, freethinking, heathen,
   heretical, hypocritical, inconstant, incredulous, infidel,
   infidelic, insincere, irreligious, minimifidian, nonbelieving,
   not true to, nullifidian, of bad faith, pagan, perfidious,
   recreant, renegade, repudiative, sceptical, shifting, shifty,
   tergiversant, tergiversating, traitorous, treacherous, treasonable,
   treasonous, trothless, two-faced, unbelieving, unchristian,
   unconfident, unconverted, unconvinced, unfaithful, unloyal,
   unreliable, unscrupulous, unstable, unsteadfast, untrue,
   untrustworthy, wavering, without faith