He Is Not Speaking Like This To Commend Himself. Indeed The Corinthians Themselves Are His Letter of Recommendation, Written By The Spirit of God (2 Corinthians 3:1-6).
He firmly points out that he does not need to commend himself to them like this, for are they not themselves a testimony to his success in Christ? They are his letters of recommendation. And he goes on to describe the wonder of what has happened to them. It is the Spirit of the living God Who has written in their hearts the new covenant sealed by the blood of Christ. They have been reborn and transformed by His activity. What they are enjoying is no outward covenant written on stone, which in the end results in failure and condemnation. It is one written by God within them which has transformed them, and it all began through the ministry of Paul. Thus can they know that he is a true Apostle of God.