John Chrysostom

"For we write none other things unto you, than what ye read or even acknowledge." For since he spoke great things of himself and seemed to be bearing witness to himself, an odious thing, he again appeals to them as witnesses of what he says. For, he saith, let no one think that what I say is a boastful flourish of writing; for we declare unto you what yourselves know; and that we lie not ye more than all others can bear us witness. …

Homily 3 on 2 Corinthians ↗

Theophylact of Ohrid

Since the apostle seemed to have said much about himself, lest anyone say this is self-praise, he says: we write to you what you read in the present epistle and what you already know even without this. For the knowledge you previously received about me does not contradict my epistles. Some, however, understood this as follows: we write to you what you read, that is, what you recall; for reading is recollection, or higher knowledge. …

Commentary on 2 Corinthians ↗

Thomas Aquinas

Then he calls on them to witness to this holy manner of life, saying, For we write you nothing but what you can read and understand. As if to say: these things I write to you are not unknown to you, because you have already read them in the first letter, and you know them by experience: "I am writing you no new commandment, but an old commandment" (1 Jn. 2:7). …

Commentary on 2 Corinthians ↗