John Calvin Commentary 2 Corinthians 10:9

"that I may not seem as if I would terrify you by my letters." — 2 Corinthians 10:9 (ASV)

That I may not seem to terrify. Again he touches on the slander which he had formerly refuted (2 Corinthians 10:2), that he was bold in his writings, while in their presence his courage failed him. On this pretext they disparaged his writings. “What!” they said, “will he terrify us by letters from a distance, while, if he were present with us, he would scarcely venture to mutter a word!” Therefore, so that his letters would not have less weight, he answers that no objection raised against him should either destroy or weaken his credibility and that of his doctrine, for deeds were not to be less valued than words.

He was not less powerful in actions when present than he was by words when absent. Therefore, it was unfair that his bodily presence should be considered contemptible. By deed, here, he means, in my opinion, the efficacy and success of his preaching, as well as the qualities that were worthy of an Apostle, and his whole manner of life. Speech, on the other hand, denotes — not the very substance of doctrine, but simply the form of it, and the bark, so to speak; for he would have contended for doctrine with greater keenness. The contempt, however, arose because he was deficient in that ornament and splendor of eloquence which secures favor.