The Oxford Thesaurus An A-Z Dictionary of Synonyms - faithless adj.

1) sceptical, doubting, unbelieving, disbelieving, agnostic, atheistic, freethinking: Faithless wretches are called Doubting Thomases. 2) unfaithful, disloyal, treacherous, traitorous, perfidious, shifting, shifty, fickle, inconstant, untrustworthy, unreliable, false, hypocritical, insincere, dishonest, false, crooked, unscrupulous, conscienceless, recreant: With faithless friends like her, who needs enemies?