John Chrysostom

"And my speech and my preaching was not in persuasive words of wisdom:" that is, had not the wisdom from without. Now if the doctrine preached had nothing subtle, and they that were called were unlearned, and he that preached was of the same description, and thereto was added persecution, and trembling and fear; tell me, how did they overcome without Divine power? …

Homily on 1 Corinthians 6 ↗

Theophylact of Ohrid

My preaching, he says, was not adorned with outward persuasiveness and eloquence, but consisted "in demonstration of the Spirit," that is, it had as its proof the Holy Spirit Himself. This means either that the apostle, through his grace-filled ministry of the Word, instilled faith in his listeners in some extraordinary manner, or that he performed signs and wonders; for he adds: "and of power," that is, of signs. Indeed, to see the dead rising was a powerful proof of the truth of the faith. …

Commentary on 1 Corinthians ↗

Thomas Aquinas

Then when he says, and my speech, he shows that he made no pretence at loftiness of speech among them. In regard to this he does three things. First, he disavows any unbecoming method of preaching when he says: and my speech, whenever I instructed anyone separately and in private: "Let no evil talk come out of your mouths, but only such as is good for edifying" (Eph 4:29), and my message, whenever I spoke in public, was not in the persuasive words of human wisdom, i.e., rhetoric, which forms phrases to persuade. …

Commentary on 1 Corinthians ↗