An epistle of Christ (επιστολη Χριστου). He turns the metaphor round and round. They are Christ's letter to men as well as Paul's.

Not with ink (ου μελαν). Instrumental case of μελας, black. Plato uses το μελαν for ink as here. See also 2 John 1:12; 3 John 1:13.

Of stone (λιθιναις). Composed of stone (λιθος and ending -ινος).

Of flesh (σαρκιναις). "Fleshen" as in 1 Corinthians 3:1; Romans 7:14.