2 Corinthians 3:1-6

You yourselves are our letter, written on our hearts, known and read by everybody.

(v2, NIV)

Those at the Corinthian church are Paul's letter of recommendation. By knowing or seeing them, they will also know what kind of person Paul is. How will you know someone? You can tell what kind of person they are by their fruit. Their fruit can be seen bye their children, those they mentor or those they disciple.

"Watch out for false prophets. They come to you in sheep's clothing, but inwardly they are ferocious wolves. By their fruit you will recognize them. Do people pick grapes from thornbushes, or figs from thistles? Likewise every good tree bears good fruit, but a bad tree bears bad fruit. A good tree cannot bear bad fruit, and a bad tree cannot bear good fruit. Every tree that does not bear good fruit is cut down and thrown into the fire. Thus, by their fruit you will recognize them.

(Matthew 7:15-19, NIV)

Continuing with the letter of recommendation idea, they are written with the Spirit.

You show that you are a letter from Christ, the result of our ministry, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.

(v3, NIV)

Such confidence as this is ours through Christ before God. Not that we are competent in ourselves to claim anything for ourselves, but our competence comes from God. He has made us competent as ministers of a new covenant—not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.

(v4-6, NIV)

Once again Paul is giving God the credit. All we have is from God and to state otherwise would be wrong.

This new covenant is of the spirit as opposed to the letter. In the times of the Old Testament they were given the law and lived by the letter of the law. This is what they had and they tried to live by it. However, now know that we are to live by the Spirit. Paul isn't necessarily pointing out one's superiority over the other, but I guess that comes in the next verses. Let us live by the Spirit and not by the letter. "For the letter kills, but the Spirit gives life" (v6, ESV)