Paul's opponents carried letters of recommendation. Paul replies that the Corinthian church itself is his letter, written on hearts and read by all. The image recalls the prophets' promise that God would one day write His law on hearts and replace hearts of stone with hearts of flesh. Paul's sufficiency comes not from himself but from God, who made him a minister of a new covenant, not of the letter but of the Spirit.
He then contrasts the two covenants with care. The old covenant, engraved on stone, came with such glory that Israel could not gaze at Moses' face. Yet that glory was fading, and the covenant brought condemnation because it exposed sin without giving power to obey. The ministry of the Spirit brings righteousness and life, and its glory is permanent and surpassing.
Paul reads Exodus 34 typologically. Moses veiled his face so Israel would not see the fading glory. A similar veil lies over hearts when the old covenant is read apart from Christ. Only in Christ is it removed. Whenever a person turns to the Lord, the veil is taken away. The Lord is the Spirit, and where the Spirit of the Lord is, there is liberty. All believers, with unveiled faces, behold the Lord's glory as in a mirror and are transformed into the same image from one degree of glory to another.