John Chrysostom

"But let each man prove his own work." Here he shows that we ought to be scrutinizers of our lives, and this not lightly, but carefully to weigh our actions; as for example, if thou hast performed a good deed, consider whether it was not from vain glory, or through necessity, or malevolence, or with hypocrisy, or from some other human motive. …

Homily on Galatians 6 ↗

Theophylact of Ohrid

Let him examine, he says, his own actions carefully (for this is what the word "let him test" means), whether he has done something out of vainglory, or out of hypocrisy, or for some other human reason, and then let him not boast before another. But if he cannot restrain himself, let him boast before himself, that is, subjecting himself to his own judgment, let him consider today's deed better than yesterday's and take pride in a good deed. …

Commentary on Galatians ↗

Oecumenius

"But let each person prove." Let each person examine his own deeds which he himself performed in life. For by doing this he will prepare himself for better things. Yet this is not the act of one who judges, but of one who humbles himself. For it is not fitting, he says, to boast; if anything, at least do not, he says, boast as the Pharisee did against your neighbor, but against yourself. (see Luke 18:9-14) — [THEODORET] In another way. …

Commentary on Galatians ↗

Jerome

But let each one test his own work, and then his reason to boast will be in himself alone and not in his neighbor. The meaning is this: You, who consider yourself spiritual and stronger in the weakness of others, should not focus on the weakness of the one who is falling but on your own strength. …

Commentary on Galatians ↗

Thomas Aquinas

Now the way to avoid such a failing is to consider one's own defects, for it is because one considers the defects of others and not his own that he seems to himself to be something in comparison to others in whom he observes defects; and not considering his own, he has a feeling of pride. …

Commentary on Galatians ↗