John Chrysostom

For since he had said, "Ye have not rather mourned, that such an one should be taken away;" and, "Purge out the old leaven;" and it was likely that they would surmise it to be their duty to avoid all fornicators: for if he that has sinned imparts some of his own mischief to those who have not sinned, much more is it meet to keep one's self away from those without: (for if one ought not to spare a friend on account of such mischief arising from him, much less any others;) and under this impression, it was probable that they would …

Homily on 1 Corinthians 16 ↗

Theophylact of Ohrid

In which epistle? In this very same one. For when he said above, "purge out the old leaven," hinting at the fornicator, as was shown, then from these words it was already evident that one must not mingle with fornicators. But since they might think that one must withdraw from all fornicators, even from those who were among the Greeks, he explains which fornicators he is speaking of.

Commentary on 1 Corinthians ↗

Thomas Aquinas

Above the Apostle had advised the Corinthians to remove a sinner from their midst. But they postponed doing this, because they gave a false interpretation to something he had written in a previous epistle. Consequently, in correcting this misunderstanding he does three things: first, he repeats what he had said in the previous epistle; secondly, he corrects the false interpretation (v. 10); thirdly, he gives the true interpretation (v. 11). …

Commentary on 1 Corinthians ↗