12:21 [And] lest... my God will humble me among you. Lest he be humiliated by the prevalence of sensual sins also.
The uncleanness and fornication and lasciviousness. The sins here named are the peculiar Gentile sins which he has rebuked so severely in both epistles.
12:21 Who had sinned before - My last coming to Corinth. Uncleanness - Of married persons. Lasciviousness - Against nature.
12:11-21 We owe it to good men, to stand up in the defence of their reputation; and we are under special obligations to those from whom we have received benefit, especially spiritual benefit, to own them as instruments in God's hand of good to us. Here is an account of the apostle's behaviour and kind intentions; in which see the character of a faithful minister of the gospel. This was his great aim and design, to do good. Here are noticed several sins commonly found among professors of religion. Falls and misdeeds are humbling to a minister; and God sometimes takes this way to humble those who might be tempted to be lifted up. These vast verses show to what excesses the false teachers had drawn aside their deluded followers. How grievous it is that such evils should be found among professors of the gospel! Yet thus it is, and has been too often, and it was so even in the days of the apostles.
