Paul now uses the word “epistle” figuratively. He had no need of literal introductory letters, for his converts were an all-sufficient proof of his apostleship. He needed no written documents to establish his apostolic authority. The metaphor of a written letter signifies both that the Corinthian believers had the word and law of God written on their hearts and that they were living epistles written on Paul’s heart. The first constituted evidence that they were true Christians, and the second that Paul was a true apostle. They were the “seal” of his “apostleship” (1 Cor. 9:2).

Textual evidence may be cited (cf. p. 10) for reading “your hearts.”