apistos (Greek #571)
(actively) disbelieving, i.e. without Christian faith (specially, a heathen); (passively) untrustworthy (person), or incredible (thing)
KJV usage: that believeth not, faithless, incredible thing, infidel, unbeliever(-ing).
Pronounce: ap'-is-tos
Origin: from 1 (as a negative particle) and 4103