'Letter' in the Bible

You are our letter, inscribed on our hearts, known and read by all people,

revealing that you are a letter of Christ, delivered by us, inscribed not with ink but with the Spirit of the living God, not on stone tablets but on tablets of human hearts.

who also makes us adequate [as] servants of a new covenant, not of the letter, but of the Spirit, for the letter kills, but the Spirit gives life.

For if indeed I grieved you by my letter, I do not regret [it]. Even if I did regret [it] (I see that that letter grieved you, even though for {a short time}),