Evidence of salvation is an epistle in itself. The salvation of the Corinthians was in Paul’s heart and in the hearts of those who were with him, for he was always thinking about it.
What means this, "ye are?" 'Did we need to be commended to others, we should have produced you before them instead of an epistle.' And this he said in the former Epistle. "For the seal of mine Apostleship are ye." But he doth not here say it in this manner, but in irony so as to make his question, "Do we need epistles of commendation?" more cutting. And in allusion to the false apostles, he added, "as do some, [epistles of commendation] to you, or letters of commendation from you" to others. Then because what he had said was severe, he softens it by adding, "Ye are our epistle, written in our hearts, known of all."
Here he testifieth not only to their love, but also to their good works: since they are able to show unto all men by their own virtue the high worth of their teacher, for this is the meaning of, "Ye are our epistle." What letters would have done to commend and gain respect for us, that ye do both as seen and heard of; for the virtue of the disciples is wont to adorn and to commend the teacher more than any letter.
"Written in our hearts." That is, which all know; we so bear you about every where and have you in mind. As though he said, Ye are our commendation to others, for we both have you continually in our heart and proclaim to all your good works. Because then that even to others yourselves are our commendation, we need no epistles from you; but further, because we love you exceedingly, we need no commendation to you. For to those who are strangers one hath need of letters, but ye are in our mind. Yet he said not merely, "ye are [in it]," but "written in [it]," that is, ye cannot slide out of it. For just as from letters by reading, so from our heart by perceiving, all are acquainted with the love we bear you. If then the object of a letter be to certify, "such an one is my friend and let him have free intercourse [with you], your love is sufficient to secure all this. For should we go to you, we have no need of others to commend us, seeing your love anticipateth this; and should we go to others, again we need no letters, the same love again sufficing unto us in their stead, for we carry about the epistle in our hearts.
What, he says, letters would have done, in which you would have commended and glorified us, that very thing you accomplish by your life of faith, which everyone sees and hears. And wherever we go, we carry you with us everywhere, because you are written in our heart, and we proclaim your virtue to all. Thus, since you are for me a letter of commendation to others, I have no need of other letters from you in order to become known to those who do not know me. Likewise, since I have you in my heart, I have no need for others to recommend me to you. Letters of commendation are needed for strangers, not for acquaintances, and you are so written in my heart that you cannot leave it. Here he testifies not only to his love for them, but also to their virtuous life, that is, that they were able to demonstrate before all people the worthiness of their teacher, for the virtue of disciples serves as an adornment for the teacher.
Therefore he at once gives the reason for this, saying, you yourselves are our letter of recommendation; as if to say: I have a good letter; I do not need others. In regard to this, he does two things: first, he shows what that letter is which he has; secondly, he explains this (v. 3). In regard to the first he does two things: first he shows what that letter is; secondly, he shows that it is sufficient for commending him (v. 3b). He says, therefore, you are our letter, i.e., the letter through which our dignity is made manifest, by which we are commended, so that we do not need other letters: "You are our glory" (1 Th. 2:20); "My little children, with whom I am again in travail, until Christ be formed in you" (Gal. 4:19). But is this letter sufficient? Yes, because it is written on your hearts. Here he touches on two things causing the sufficiency of such letters. One is that it should be understood and known by the one for whom it is sent; the other that he still seeks, and not that he knows himself to have it. As to this he says, written on our hearts, because we always have you in mind, having a special care for you: "I hold you in my heart" (Phil. 1:7). The other is that he to whom it is sent may read and know it; hence, he says, to be known and read by all men. To be known, I say, because you have been instructed and converted by us; but it is read, because by our example even others imitate you: "Write the vision; make it plain upon tablets, so he may run who reads it" (Hab. 2:2).
[AD 384] Ambrosiaster on 2 Corinthians 3:2