John Chrysostom
See verse 1. "You," he says, "are our letter" (v. 2). What does "You are our letter" mean? "If we need to recommend ourselves before others, we shall place you in the midst instead of a letter of approval."
He said the same in the first epistle: "You are the seal of my apostleship in the Lord" (1 Cor 9:2).
But here he did not speak simply. He spoke with a certain irony, to give his speech greater force: "Do we need," he says, "letters of approval to you or from you?"
Having the false apostles in mind, he added: "as some do, letters of approval to you or from you, to others."
Then, because what he said was hard to hear, he softens his words further, saying: "You are our letter, written in our hearts, known and read by all men. You show yourselves to be a letter of Christ" (vv. 2-3).
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Here he expresses not only his love for them, but also testifies to their virtuous life. That is, by their virtues they can prove before all the dignity of their teacher.
This is the meaning of the words: "You are our letter." What your letters would have done, if you had approved and praised us in them, you yourselves accomplish by your life according to faith, which all see and hear about.
The virtues of disciples serve as the best adornment for their instructor, and approve him better than any letter.
"Written in our hearts" means that it is known to all, because we carry you with us everywhere and keep you in our heart.
It is as though he said: "You serve as our approval before others. We always have you in our heart, and before all we proclaim your virtues.
Therefore we not only have no need of letters of approval from you to others, because you serve as our approval, but even for you yourselves we need no testimony from others, because we love you deeply.
Letters of approval are needed for strangers, but you are in our hearts." He did not simply say, "you are," but "you are written," that is, so that you cannot be erased from our hearts.
As by reading our letters, so by being assured from our hearts, all know the love we have for you.
If letters are used to show that such a person is my friend and enjoys my confidence, then your love replaces all this for us.
Therefore, whether we go to you, we do not need to take approval from others, since your love for us fully replaces it.
Or whether we go to others, again we do not need to take letters of approval from you, because instead of them this same love is sufficient for us here also.
We carry the epistle in our hearts.