Also see definition of "Unbeliever" in Word Study
Study Dictionary
Unbeliever
UNBELIEVER [ISBE]
UNBELIEVER - un-be-lev'-er: This word follows closely the lines of "unbelief" (which see) in its relation to originals. Once only (Acts 14:2) it represents the participle apeithountes, "disobeying (ones)." Elsewhere (nine cases) it represents apistos, "faithless," "without faith." In six of these passages (all in 1 and 2 Corinthians) it denotes the unconverted pagan as distinguished from the convert. In the other passages (Lk 12:46; Tit 1:15; Rev 21:8) the reference is to the unbelief which comes of moral resistance to God.
Also see definition of "Unbeliever" in Word Study