You are our letter, written in our hearts, known and read by all men; being manifested that you are a letter of Christ, cared for by us, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts. [2 Corinthians 3:2-3]
The letter of commendation is somewhat like the referral letter. Apparently some people were questioning Paul’s lack of such letters while some teachers had such letters to show. Paul explained that the Corinthian church would be his letter of commendation because the behavior and conduct of the church members under his ministry would be the public testimony for his character. In fact, the church would be Christ’s letter of commendation to all men.
People outside the faith often evaluate the faith based on the behavior and conduct of its members. When a member of the faith does something bad, the faith gets attacked. When a member of the faith does something good and attributes it to God, people would have a more positive view. This is something members of the faith ought to note.