John Chrysostom

"For not he that commendeth himself is approved, but whom the Lord commendeth." He did not say, we are so, "but whom the Lord commendeth." Seest thou how modestly he speaks? But if as he proceeds he stirreth up loftier words, wonder not, for this also cometh of Paul's prudence. For if he had gone on in every part to speak lowly words, he would not have hit these men so effectually, nor have extricated the disciples from their error. …

Homily 22 on 2 Corinthians ↗

Theophylact of Ohrid

He did not say: we are skillful, but: "whom the Lord commends," that is, the one whom the reality of labor and endeavor, by the grace of God, has shown to be worthy.

Commentary on 2 Corinthians ↗

Thomas Aquinas

For it is not the man who commends himself that is accepted, i.e., approved by God or by men: "Let another praise you, and not your own mouth; a stranger, and not your own lips" (Prov. 27:2), but the man whom the Lord commends, i.e., makes commendable by good works and miracles. For God is the cause of the entire good done by me.

Commentary on 2 Corinthians ↗