Do we begin again to commend ourselves? Or do we need, as some others, epistles of commendation to you or letters of commendation from you? 2 You are our epistle written in our hearts, known and read by all men; 3 clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart. — 2 Corinthians 3:1-3.
Paul refers to letters of introduction, often used to assure new churches that those coming to them are authentic Christians. Initially, the false apostles commended themselves. Paul says he needs no letter of commendation when coming to the Corinthian church, because they themselves are his letter of commendation. They are saved because he has been there with the gospel. [The Bible Panorama.]