What does 2 Corinthians 3:3 mean?
Paul should not need to prove, any further, that he is a genuine apostle, truly sent by God to represent Christ. He has the only letter of recommendation he needs: the genuine transformation in the Corinthians themselves. These people were changed from idol-worshiping pagans into followers of Jesus. They are Paul's letter of recommendation, written on his own heart and open to be read by all.Paul is not claiming that he is the one who made the Corinthian believers into Christians. He is the representative. He merely delivered the letter, but it was written by Christ with the Holy Spirit of the living God, instead of ink. And it was written onto human hearts, instead of stone tablets.
Paul's point is that the Corinthians' faith is genuine. Christ indisputably wrote Himself into their hearts through the Holy Spirit. All of that was real. Since Paul is the one who brought that "letter" to them, they should see clearly that he is truly a genuine apostle sent by God.