Jamieson, Fausset and Brown on 1 Corinthians 5:12

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 (Ro 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.