The Greek adjective apistos means unbelieving, faithless, or incredible. As a noun it refers to an unbeliever — someone outside the Christian faith. It appears approximately 23 times in the New Testament and is particularly prominent in Paul's letters addressing mixed marriages and the church's relationship to the broader pagan world.
Apistos is the defining category for those outside faith in Christ. Paul uses it frequently in 1 Corinthians: 'Do not be yoked together with unbelievers (apistois)' (2 Corinthians 6:14) — not a command to social isolation, but a warning against forming fundamental covenants or partnerships with those whose foundational commitments are incompatible. Yet Paul also has a pastoral theology of apistoi. In 1 Corinthians 7:12–16, he addresses couples where one spouse is a believer and the other is not — the believing spouse is not to divorce the unbelieving partner, for the unbeliever is 'sanctified' through the believer. This remarkable passage shows that God works through the faithful believer even in relationships with apistoi. First Peter 2:12 commands believers to 'live such good lives among the pagans that, though they accuse you of doing wrong, they may see your good deeds and glorify God on the day he visits us.' The goal is not separation from apistoi but witness to them.