John Calvin Commentary 2 Corinthians 13:9

"For we rejoice, when we are weak, and ye are strong: this we also pray for, even your perfecting." — 2 Corinthians 13:9 (ASV)

For, we rejoice. Either the causal particle γὰρ (for,) must be taken as meaning—therefore; or it is a second reason why he does not refuse to be regarded as a reprobate—for their sake, and with a view to their advantage. Let the reader select whichever he chooses, for it is of no consequence. When he says, Provided you are strong, I shall willingly submit to be reckoned weak, there is an antithesis in the words—not in the meaning; for weakness means here, as formerly (2 Corinthians 13:4), contempt. On the other hand, he means that the Corinthians will be strong, if they are full of the power and grace of God.

And this also, He now repeats what he had already stated several times: that out of necessity—not from his own inclination—he was more severe than they would have wished. Further, by this means, too, he spared them, so that he might not be forced to resort to more severe measures when he was present with them.

The perfection, of which he speaks, consists in a fit proportion and sound condition of all the members. He now alludes to good physicians, who cure particular diseases in such a way as not to mutilate any part of the body. Since he is concerned to secure a perfection of this nature, he says that for this reason he takes precautions against the necessity of resorting to more severe measures. For we see that those who at first shrink back from the slight pain or uneasy feeling of a plaster are eventually forced to endure the torture of burning or amputating—and that, too, when the outcome is extremely doubtful.