Jamieson, Fausset and Brown on 2 Corinthians 12:21

21. my God--his God, however trying the humiliation that was in store for him.

will humble me--The indicative implies that the supposition will actually be so. The faithful pastor is "humbled" at, and "bewails" the falls of his people, as though they were his own.

sinned already--before my last coming [Bengel], that is, before the second visit which he paid, and in which he had much at Corinth to rebuke.

have not repented--shall not have repented [Alford].

uncleanness--for example, of married persons (1Th 4:7).

fornication--among the unmarried.