John Chrysostom

"If I must needs glory, I will glory of the things which concern my weakness." Seest thou that he no where glorieth of miracles, but of his persecutions and his trials? For this is meant by "weaknesses." And he shows that his warfare was of a diversified character. For both the Jews warred upon him, and the Gentiles stood against him, and the false brethren fought with him, and brethren caused him sorrow, through their weakness and by taking offense:-on every side he found trouble and disturbance, from friends and from strangers. …

Homily 25 on 2 Corinthians ↗

Theophylact of Ohrid

By "weakness" he means persecutions; for in hunger, scourging, shipwreck, and the endurance of other calamities, human weakness is revealed. He does not mention signs here, because they were a gift of God; but the calamities, together with the power of God, also demonstrated his patience.

Commentary on 2 Corinthians ↗

Thomas Aquinas

Then when he says, if I must boast, he confirms some of the statements made above. Or say that above he speaks about the evils he commendably suffered; but here about the evils he prudently avoided. But because the avoidance of evils that arise against the faith seems to imply weakness, therefore, first, he states that he wishes to glory in those that imply weakness; secondly, he proposes an oath to confirm his statements (v. 31); thirdly, he shows how he avoided some evils (v. 32). …

Commentary on 2 Corinthians ↗