John Chrysostom

And "tell me not," saith he, "that the fornicator is but one: the charge hath become common to all." Wherefore at once he added, "and ye are puffed up:" he said not, "with the sin;" for this would imply want of all reason: but with the doctrine you have heard from that person. This however he set not down himself, but left it undetermined, that he might inflict a heavier blow. And mark the good sense of Paul. …

Homily on 1 Corinthians 15 ↗

Theophylact of Ohrid

It would have been fitting, he says, to weep, because the reproach spread to the entire Church; it would have been fitting to pray with tears, as over a disease and contagion, "that he might be taken away from among you," that is, that he be cut off from you, as a public evil. Again here too he did not mention the fornicator by name, but said "he who has done such a deed."

Commentary on 1 Corinthians ↗

Thomas Aquinas

Then when he says, you are arrogant, he mentions the guilt of those who condoned this sin: first, he condemns them for condoning it; secondly, he supplies what they failed to supply (v. 3). In regard to the first he detects three vices: first, pride, when he says: You are arrogant [puffed up], namely, with the wind of pride, for considering yourselves innocent as compared with the sinner, just as the Pharisee who said: "I am not as the rest of men…or even as this tax collector" (Lk 18:11); "He will dash them puffed up and speechless …

Commentary on 1 Corinthians ↗