Also see definition of "Faithless" in Word Study
Study Dictionary
Faithless
FAITHLESS [ISBE]
FAITHLESS - fath'-les: The translation of apistos, "without faith," having the sense of "unbelieving," "disbelieving." Jesus upbraids the people, "O faithless and perverse generation!" (Mt 17:17; Mk 9:19; Lk 9:41); He says to Thomas, "Be not faithless, but believing" (Jn 20:27); the Revised Version (British and American) adds, "If we are faithless," instead of "believe not" (2 Tim 2:13); compare 1 Cor 7:12-15; 10:27; 14:22,24, etc.; Tit 1:15. In Lk 12:46 apistos has the sense of "unfaithful," so the Revised Version (British and American); perhaps also Rev 21:8, "unbelieving."
Also see definition of "Faithless" in Word Study