v1-3: Some 'false apostles' carried letters of commendation to demonstrate their authority. In contrast, Paul pointed to the church at Corinth itself. The proof of apostleship was their changed lives; this demonstrated his authority, and the power and authority of the gospel of Christ. Such letters written on "tablets of human hearts" could only be the work of God.
v4: Paul's confidence is indeed that the work of God is true.
v5-6: Paul had already asked the questions, "Who is equal to such a task?" or 'Who is sufficient for these things?' Any ministry of Christ is beyond the ability of natural man. Our ministry is God's calling, and He will give us strength; our competence and sufficiency are from God.
Paul uses the word translated competent, or sufficient (Gk: HIKANOO), to be enough, to render fit (not the same as power, Gk: DUNAMIS) – this carries the idea that God equips us for the task to which He has called us.
The calling is consistent with the message. The message is of the new covenant, of the Holy Spirit, of life. Our calling is to the proclamation of the message. The new covenant is to do with spirit, not the letter; mercy not law; as such it is life.
v7-8: In the O.T. the people followed the ministry of "death", the law, written on stones. Even this message had great glory, being from God, since the people saw that Moses' face shone (Ex 34.29-35). How much greater, therefore, is the ministry of life through the Spirit!
v9: The message is the ministry of righteousness, not condemnation. Again, it is glorious.
v10-11: The old covenant is fading away, Heb 8.13. The new covenant, in Christ, will last, and will not be replaced. It has "surpassing glory", and glory "which lasts".
v12-13: We therefore have great boldness; confidence in the power and relevance of the message. The theme here is similar to the 'better' theme of the letter to the Hebrews. Moses covered his face; but that radiance was fading away. Although Moses delivered the law to the people, his own life was one of faith.
v14-16: The message of the O.T. is veiled, not because it is incorrect, but because it is fulfilled in Christ. The veil is taken away in Christ. The veil was, in reality, over their hearts, to prevent them from understanding and seeing. In Christ this veil is removed, so that we might know the only true God.
v17: The Spirit of the Lord comes in; He brings freedom and access. The contrast is of legalism in the letter (v6), written and engraved of stones (v7).
v18: We behold, with unveiled face, the glory of the Lord; how little we experience this! The glory of Christ is something we experience in ever-increasing measure. As we get to know Him more, we are changed into His likeness. We can be changed in habits and actions, in thoughts and words. Our flesh needs to be trained to become like our nature, which is now clean.