Paul has been speaking about his ministry, and he knows how that might sound. Barnes explains that he asks this to meet an objection: after speaking of his triumph and sincerity, it might seem he was praising himself. So he raises the charge before anyone else can. His second question presses further. Others had come to Corinth with letters of recommendation. Does Paul, who brought them the gospel, need a letter to prove who he is? Gill says these questions strike at the false apostles who carried such letters from place to place, "having nothing truly good and excellent in them to recommend them to others."

Paul answers both questions at once: "Ye are our epistle written in our hearts, known and read of all men" (2 Corinthians 3:2). The Corinthians themselves are "the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God" (2 Corinthians 3:3). He does not need a letter, because their changed lives speak for him. And he gives the credit to God: "Not that we are sufficient of ourselves ... but our sufficiency is of God" (2 Corinthians 3:5).

Letters of commendation were a common way for early believers to introduce a traveling teacher to a church that did not know him. Acts 18:27 records one: when Apollos went to Achaia, "the brethren wrote, exhorting the disciples to receive him." Barnes says such letters were "sometimes given by Christians, and are by no means improper," though they "do not appear to have been sought or used by the apostles generally." Paul returns to the subject of commending himself later in the letter: "we commend not ourselves again unto you" (2 Corinthians 5:12).

Paul's answer moves the proof of ministry from paper to people. The best evidence that God was at work through him was not a document but a church: lives written on "with the Spirit of the living God" and "in fleshy tables of the heart" (2 Corinthians 3:3). Matthew Henry says, "Nothing is more delightful to faithful ministers, or more to their praise, than the success of their ministry, as shown in the spirits and lives of those among whom they labour."

The question also shows Paul's humility. Matthew Henry notes that "even the appearance of self-praise and courting human applause, is painful to the humble and spiritual mind." Paul is uncomfortable defending himself, so he points away from himself to what God has done in the Corinthians, and then to God, who alone makes anyone sufficient (2 Corinthians 3:5-6).