Strong's G571 — ἄπιστος (adjective)
apistos (ap'-is-tos) adj.
1. (actively) disbelieving, i.e. without Christian faith
2. (specially) a heathen
3. (passively) untrustworthy (person) or unbelievable, incredible (thing)
[from G1 (as a negative particle) and G4103]
KJV: that believeth not, faithless, incredible thing, infidel, unbeliever(-ing) 
Root(s): G1, G4103 
Hebrew root: πιστός
Occurrences: 23