John Chrysostom
"By the mouth of two or three witnesses every word shall be established." So here also he adds, "When I come again, I will not spare." These words mean: "I will no longer postpone punishment if, God forbid, I find you uncorrected. I will certainly punish, and I will fulfill what I said."
His words have this meaning: once and again I spoke when I came, and now I speak in writing. If you obey me, what I so desired has been fulfilled. But if you do not obey, then in the end it is necessary to do what was said and apply punishment.
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The Apostle first showed that he cannot do this unless compelled. He called the necessity of punishment weeping and humiliation, saying: "That again, when I come, my God may humble me among you, and that I may mourn for many who sinned before and did not repent" (2 Cor 12:21).
He even defended himself before them, saying that he had already warned them once, twice, and three times. He does and devises everything in order to avoid punishment and make them better by frightening them only with words.
Only then does he utter these dreadful and fearful words: "When I come again, I will not spare." He did not say, "I will avenge, I will chastise, I will demand punishment."
Even when speaking of punishment itself, he again speaks in a fatherly way. He shows his mercy and compassion toward them, because of which he has always spared them and deferred punishment.