“You yourselves are our letter, written on our hearts, known and read by everyone, revealing that you are a letter of Christ, delivered by us, written not with ink but by the Spirit of the living God, not on stone tablets but on tablets of human hearts.”
2 Corinthians 3.2-3
How did Paul recommend himself to the Corinthians? By the converts made. His credentials consisted of the gospel in the hearts of saved men and women.
People like to have their names on university or hospital buildings. They love diplomas hanging on walls. The true measure of a servant of Christ is work done among people for salvation.