Albert Barnes on 2 Corinthians 10:9

That I may not seem, etc. The meaning of this verse seems to be this: "I say that I might boast more of my power in order that I may not appear disposed to terrify you with my letters merely. I do not threaten more than I can perform. I have it in my power to execute all that I have threatened,, and to strike an awe not only by my letters, but by the infliction of extraordinary miraculous punishments. And if I should boast that I had done this, and could do it again, I should have no reason to be ashamed. It would not be vain empty boasting; not boasting which is not well-founded."