2 Corinthians 3:1 Do we begin again to commend ourselves? Or do we need, as some others, epistles of commendation to you or letters of commendation from you? 2 You are our epistle written in our hearts, known and read by all men; 3 clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart.
"The thirteenth canon of the Council of Chalcedon (A.D. 451) ordained that 'clergymen coming to a city where they were unknown, should not be allowed to officiate without letters commendatory from their own bishop.' The history (Ac 18:27) confirms the existence of the custom here alluded to in the Epistle: 'When Apollos was disposed to pass into Achaia [Corinth], the brethren [of Ephesus] wrote, exhorting the disciples to receive him.' This was about two years before the epistle...” (Jamieson-Fausset-Brown) Paul "means that the change which had been produced in their hearts and lives, in their dispositions, words, and actions, by his ministry, and that of his fellow-labourers, a change which could not have been effected except by the power of God, was a demonstration that God had sent them, and was present with them, giving efficacy to the word of his grace, a letter written in our hearts." (Benson Commentary)
4 And we have such trust through Christ toward God. 5 Not that we are sufficient of ourselves to think of anything as being from ourselves, but our sufficiency is from God, 6 who also made us sufficient as ministers of the new covenant, not of the letter but of the Spirit; for the letter kills, but the Spirit gives life.
Paul "hastens to insist that whatever he has done is not his own work but the work of God. It is God who has made him adequate for the task which was his. It may be that he is thinking of a fanciful meaning that the Jews sometimes gave to one of the great titles of God. God was called El Shaddai... , which is The Almighty, but sometimes the Jews explained El Shaddai to mean The Sufficient One. It is he who is all-sufficient who has made Paul sufficient for his task.” (William Barclay)
7 But if the ministry of death, written and engraved on stones, was glorious, so that the children of Israel could not look steadily at the face of Moses because of the glory of his countenance, which glory was passing away, 8 how will the ministry of the Spirit not be more glorious? 9 For if the ministry of condemnation had glory, the ministry of righteousness exceeds much more in glory. 10 For even what was made glorious had no glory in this respect, because of the glory that excels. 11 For if what is passing away was glorious, what remains is much more glorious.
God's Commandments could not make them alive to God. And the sacrificial system could not permanently take away sin. (Heb 120:4) Only the Eternal Son of God could do these things. This temple system "could not introduce them to union with him, and its carnal ordinances and worldly promises could not render them spiritually minded. Thus the letter, that external, emblematical, and shadowy dispensation, killed such as adhered to it, and rejected the gospel; but the Spirit giveth life. As the Spirit of God is the grand promise of the new covenant, (see Isaiah 44:3; Isaiah 59:21; Joel 2:28; John 7:37-38,) so by this the gospel doctrines, precepts, and promises, are made spirit and life to us; repentance unto life and living faith are begotten in us, the favour of God is manifested, and union with God imparted, productive of a spiritual mind, which is life and peace.” (Benson)
12 Therefore, since we have such hope, we use great boldness of speech— 13 unlike Moses, who put a veil over his face so that the children of Israel could not look steadily at the end of what was passing away. 14 But their minds were blinded. For until this day the same veil remains unlifted in the reading of the Old Testament, because the veil is taken away in Christ. 15 But even to this day, when Moses is read, a veil lies on their heart. 16 Nevertheless when one turns to the Lord, the veil is taken away. 17 Now the Lord is the Spirit; and where the Spirit of the Lord is, there is liberty. 18 But we all, with unveiled face, beholding as in a mirror the glory of the Lord, are being transformed into the same image from glory to glory, just as by the Spirit of the Lord.
"The end of this was Christ. The whole Mosaic dispensation tended to, and terminated in, him. But the Israelites had only a dim wavering sight of him, of whom Moses spake in an obscure, covert manner…. But their minds were blinded — Besides the obscurity of that dispensation, there was evidently blindness on their minds. They rested in the outward letter, and did not understand or apprehend the spiritual sense of the law.” (Benson)
Nevertheless when one turns to the Lord, the veil is taken away. "The blindness and prejudice of the human heart can only be removed by the grace of God, giving a teachable temper; and when they turn to the Lord the veil shall be removed. Thus Moses, shining with the divine glory, veiled his face when he addressed the elders: but on turning to speak to the Lord, he took away the veil. Happy figure of the removal of the veil which covered the law, by the superior glory of grace. Happy transition from the bondage of the legal shadows to the glorious liberty of Zion, whose children have received the spirit of adoption, crying, Abba, Father.
The glimpse of glory which Moses saw, then a singular favour, is now open to every believing soul in regard of sanctification. The gospel is the speculum or glass, setting forth the glory of Christ. When we contemplate his lustre, the whole soul becomes irradiated with the light of life; when we behold his love, the heart presently burns with celestial fire; and as it is the property of fire to convert every substance into its own element, so we are changed into the same image, by the renovating power of the Holy Ghost. Thus Stephen before the council had a countenance irradiated with the glory of Christ, for heaven at that moment opened to his view with brighter and still brighter beauty. But whence proceeds this astonishing power of faith, or of beholding the glory of the Lord. It is because he has been pleased to make faith the grand condition of justifying and of sanctifying grace; and because it is consonant to the moral nature of things. If I look at the glory of this world, and covet its vanities, I become base by leaving God, and choosing corruption. But when looking simply and constantly at his glory, I become elevated in faith, ennobled in sentiment, and sanctified in habit. Thus faith purifies the heart, and inexplicably transforms the soul into the image of God. What then must be the change induced on glorified spirits, when they shall see him as he is.” (Sutcliffe Commentary)