John Chrysostom

That is, to those that are over the Church. Yet the Lord enjoins nothing of this sort to be observed towards those who are without the Church, such as He does in reproving a brother. Of those that are without He says. If any smite thee on the one cheek, offer to him the other also. (Mat. 5:39.) as Paul speaks, What have I to do to judge them that are without? (1 Cor. 5:12.) But brethren he bids us reprove, and turn away from.

Catena Aurea by Aquinas ↗

Augustine of Hippo

But "if he does not listen," that is, if he chooses to justify his sin as if it were a just action, "take one or two others along with you, that every word may be confirmed by the evidence of two or three witnesses. If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector." Don't consider him now in the number of your brothers. But not even so is his salvation to be neglected. …

Sermon 82.7 ↗

Jerome

But if your brother sins against you, go and confront him between you and him alone. If he listens to you, you have gained your brother. But if he does not listen, take one or two more with you, so that by the testimony of two or three witnesses every word may be confirmed. If he refuses to listen to them, tell it to the church. And if he refuses to listen even to the church, let him be to you as a Gentile and a tax collector. …

Commentary on Matthew ↗

Thomas Aquinas

Hence he says, and if he will not hear them, tell the Church. Here a denunciation is set forth. And first, a denunciation; secondly, a sentence is given; thirdly, its efficacy. The second is at and if he will not hear the Church, etc.; the third at amen I say to you, etc. He says and if he will not hear them, tell the Church, i.e., the whole community, so that he may be put to shame, so that he who was unwilling to be corrected without shame may be corrected with shame. …

Commentary on Matthew ↗