The figure which follows is freely and somewhat loosely worked out, and presents different faces in rapid succession. The figure itself is that of a commendatory letter representing the Corinthian Church: “Ye are our letter.” This figure is carried out in three directions: 1. As related to the apostles' own consciousness. The Corinthian Church is a letter written on the apostles' hearts. Their own consciousness testifies that that Church is the fruit of a divinely accredited, honest, and faithful ministry. 2. As related to the Corinthians themselves. The Church needs no letter to commend the apostles to it. It is its own commendation. As the visible fruit of the apostles' ministry they are a commendatory letter to themselves. If the question arises among them, “Were Paul and his colleagues duly commissioned?” - the answer is, “We ourselves are the proof of it.” 3. As related to others outside of the Corinthian Church. The answer to the charge that the Corinthians have been taught by irregular and uncommissioned teachers is the same: “Behold the fruit of their labors in us. We are their commission.”
At this point the figure again shifts; the letter being now conceived as written on the Corinthians' hearts, instead of on the hearts of the apostles: written by Christ through the apostles' ministry. This suggests the comparison with the law written on tables of stone, which are used as a figure of the heart, fleshy tables, thus introducing two incongruities, namely, an epistle written on stone, and writing with ink on stone tables.
Written in our hearts
See above. Compare Plato: “I am speaking of an intelligent writing which is graven in the soul of him who has learned, and can defend itself” (“Phaedrus,” 276).