John Chrysostom

But if he doth not display His power nor punish, (for thenceforward the Apostle transferred what he said from himself to Christ, thus making his threat more fearful,) it is not from weakness; for He can do it: but from long suffering. Let none then think His forbearance to be weakness. For why marvellest thou that He doth not now proceed against sinners, nor in his forbearance and long suffering exacts satisfaction, seeing that He endured even to be crucified, and though suffering such things punished not?

Homily 29 on 2 Corinthians ↗

Theophylact of Ohrid

He says this with great indignation to those who despised him as a weak and contemptible man. He speaks as if to say: since you desire to test whether Christ dwells in me, and you mock me as a weak man who does not have Christ within him, you will truly learn this if you do not correct yourselves. He shows that the words he speaks are spiritual, and that one should fear his threat as if Christ Himself had uttered it. …

Commentary on 2 Corinthians ↗

Thomas Aquinas

Then, lest they belittle the Apostle's power, he discloses his judicial power, saying: Do you desire proof that Christ is speaking in me? Here he does three things. First, he shows that he has delegation and power to judge from Christ; secondly, he shows Christ's power (v. 3b); thirdly, he shows that Christ's power is distributed to others. He says therefore: if I come, I shall not spare; rather, I shall judge most severely. …

Commentary on 2 Corinthians ↗