1 Corinthians 5
A case of sexual immorality—a man living with his father's wife—has arisen, and the Corinthians have boasted rather than mourned, failing to remove the one who did this thing from their midst. Paul issues a directive: deliver this man to Satan for the destruction of the flesh, that his spirit may be saved on the day of the Lord, a severe but redemptive measure. He invokes the Passover: clean out the old leaven, for Christ our Passover lamb has been sacrificed; celebrate the feast with unleavened bread of sincerity and truth, not with the old leaven of malice and evil. Paul clarifies a previous letter he wrote warning against associating with the sexually immoral: he meant those inside the church, not those outside the world, for you are not to judge those outside but only those inside. If someone inside is immoral or a drunkard or swindler, you must not even eat with such a person. God judges those outside; the assembly judges those inside.
1 Corinthians 5:10
Not at all meaning the people of this world who are immoral, or the greedy and swindlers, or idolaters. In that case you would have to leave this world — Paul clarifies: if he meant worldly sinners (hoi porni ho tou kosmou), the Corinthians would have to withdraw from society entirely (exit the world). That is not feasible or commanded. Christians live in a pagan world; contact with sinners is unavoidable and proper.
1 Corinthians 5:11
But now I am writing to you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a person do not even eat — 'anyone who calls himself a brother' (ei tis adelphos onomazomenos) is the key: the prohibition targets professing believers engaged in serious sin. 'Not even eat' (mēde synesthienai) means no table fellowship, the mark of community. Excommunication is signal that the person has severed themselves from the body.
1 Corinthians 5:12
What business is it of mine to judge those outside the church? Are you not to judge those inside? — 'Those outside' (exō) are non-believers; judging them is God's domain (cf. v. 13, citing Deut 17:7). 'Those inside' (esō) are church members; the church's responsibility is internal discipline. Paul redirects the Corinthians' focus: stop elevating external judges (pagan courts, ch. 6); establish just judgment within the community.
1 Corinthians 5:13
God will judge those outside. 'Expel the wicked person from among you' — the quotation from Deuteronomy 17:7 commands removal of those who commit serious sin. 'Wicked person' (ponēros) does not mean anyone who sins but one who scandalously transgresses and refuses to repent. Expulsion is the church's exercise of keys: binding and loosing through discipline and restoration. The incestuous man must be removed; his spirit's salvation rests on humiliation and (potentially) repentance.