I Corinthians 5:12

what have I to do--You might have easily understood that my concern is not with unbelievers outside the Church, but that I referred to those within it. also--Implying, Those within give me enough to do without those outside. do not ye, &c.--Ye judge your fellow citizens, not strangers: much more should I [BENGEL]. Rather, Is it not your duty to judge them that are within? God shall judge them that are without: do you look at home [GROTIUS]. God is the Judge of the salvation of the heathen, not we (Rom 2:12-16). Paul here gives an anticipatory censure of their going to law with saints before heathen tribunals, instead of judging such causes among themselves within.

In this chapter the apostle, I. Blames them for their indulgence in the case of the incestuous person, and orders him to be excommunicated, and delivered to Satan (Co1 5:1-6). II. He exhorts them to Christian purity, by purging out the old leaven (Co1 5:7, Co1 5:8). And, III. Directs them to shun even the common conversation of Christians who were guilty of any notorious and flagitious wickedness (Co1 5:9 to the end).

5:12-13 Christians are not called to judge (i.e., discipline) sin in unbelievers, but in believers.