What does 1 Corinthians 5:13 mean?

Paul's final word on the issue of Christians judging unbelievers for their sin is simple: it is not our responsibility. He states unequivocally that God judges those outside of the community of Christ (Acts 17:30).

On the other hand, Christians must accept our responsibility to judge each other when one who claims to be in Christ is participating in ongoing and unrepentant sin. The word "repentance" seems to be key to this conversation. In this specific example, the man who was having an affair with his father's wife was continuing in his sin (1 Corinthians 5:1–5). He had not confessed it as sin or turned from it. He was not even reported to have been struggling to give up sin he knew to be wrong. He continued in obvious and ongoing sin openly and willfully.

Paul applies a common phrase from the law of Moses in Deuteronomy: "Purge the evil person from among you." Christian churches must not allow such ongoing sin to remain unaddressed in the community. Doing so poisons the spiritual health of the congregation (Jude 1:12; 2 Peter 2:1) and invites slander from the unbelieving world (1 Peter 2:12; 2 Peter 2:2).

Paul put it this way in 2 Thessalonians 3:14–15: "If anyone does not obey what we say in this letter, take note of that person, and have nothing to do with him, that he may be ashamed. Do not regard him as an enemy, but warn him as a brother."