What does 2 Corinthians 3:2 mean?

Some in Corinth had apparently questioned Paul's legitimacy as an apostle. How does an apostle, truly sent by God to represent Christ, prove he is genuine? In the ancient world, some teachers carried commendations describing their own experiences, or letters of recommendation from other qualified people.

Paul, though, insists he has an even better letter of recommendation: the Corinthian Christians themselves. The transformation in them, from pagans to followers of Jesus, is all the evidence he will ever need that his ministry as Christ's representative is the real thing. The Corinthians, above all, should be able to declare that Paul's ministry is valid. Otherwise, they would be saying that their own conversion to faith in Christ was not valid.

Paul first points not to their hearts but to his own. He has expressed his deep love for the Corinthians on multiple occasions. Here, he says they have been written on the hearts of him and of his co-workers in ministry. All who know Paul can read this letter describing his affection for the Corinthians. He also describes the transformation that is taking place among them by the power of the Holy Spirit. An obvious and positive change has taken place in him, as well as in them.