2 Corinthians 3:3

3:3 revealing that you are a letter of Christ, delivered by us, written not with ink but by the Spirit of the living God, not on stone tablets but on tablets of human hearts.

2 Corinthians 4:13

4:13 But since we have the same spirit of faith as that shown in what has been written, “I believed; therefore I spoke,” we also believe, therefore we also speak.

2 Corinthians 7:8

7:8 For even if I made you sad by my letter, I do not regret having written it (even though I did regret it, for I see that my letter made you sad, though only for a short time).