See on the kindred verb repent, Mat 3:2, and compare note on Mat 21:29. Repentance is different from regret of 2Co 7:8, indicating a moral change, as is shown by the next clause.
Ye might receive damage (ζημιωθῆτε)
Rev., might suffer loss. See on Mat 16:26; see on Luk 9:25. This somewhat obscure sentence means that the salutary moral results of the apostle's letter compensated for the sorrow which it caused. The epistle which won them to repentance was no damage to them.