2 Corinthians 3:2

You yourselves. The existence of the messianic community at Corinth is the only credential which Paul needs! Written on our hearts. MacKnight says it is best to understand this as the letter itself written on the heart of Paul, and a copy of that same letter written on the hearts of the Corinthians. To know and read. MacKnight thinks the letter written on Paul's heart was the spiritual gifts which he had as an apostle (compare 1 Thessalonians 1:5 and note). But we can also understand this to mean that the whole world can know and read Paul's work through the church at Corinth!