John Chrysostom

"Wherefore show ye unto them, to the person of the churches, the proof of your love, and of our glorying on your behalf." 'Now show,' he saith, 'how ye love us; and how we do not lightly nor vainly boast in you: and this ye will show, if ye show forth love towards them.' Then he also makes his words more solemn, by saying, "unto the person of the churches." He means, to the glory, the honor, of the churches. 'For if ye honor them, ye have honored the churches that sent them. …

Homily 18 on 2 Corinthians ↗

Theophylact of Ohrid

Now, he says, show how you love us and how we do not boast of you in vain. And you will show this if you show them love. For what you do for them, you do "before the face of the churches," that is, for the honor of the churches; for they represent the face of the churches that sent them.

Commentary on 2 Corinthians ↗

Thomas Aquinas

Then when he says, "so give proof," he recommends those messengers to the Corinthians, saying: because I have sent such men to you, "so give proof, before the churches, of your love and of our boasting about you to these men," i.e., show by your deeds that you have charity toward them, and that he has truthfully commended you, and that he was right in boasting about you, before the churches, where I have been and to whom I have preached; or: before the churches, because what you do to them will be made known to all the churches.

Commentary on 2 Corinthians ↗