Paul asks whether he needs, like some, letters of recommendation. The Corinthians themselves are his letter, written on his heart, known and read by all, and they are a letter from Christ, written not with ink but with the Spirit of the living God, not on tablets of stone but on tablets of human hearts.
Paul has such confidence toward God through Christ. Not that they are sufficient in themselves: their sufficiency is from God, who made them ministers of a new covenant, not of the letter but of the Spirit, for the letter kills but the Spirit gives life.
If the ministry of death, carved in letters on stone, came with glory, so that the Israelites could not gaze at Moses' face because of its fading glory, will not the ministry of the Spirit be more glorious? If the ministry of condemnation had glory, the ministry of righteousness exceeds it in glory. What was glorious has no glory now in comparison with the surpassing glory. If what was fading away came with glory, much more will what remains be in glory.
So Paul acts with great boldness, not like Moses, who put a veil over his face. To this day, when the old covenant is read, the same veil remains over the minds of many, and it is removed only in Christ. When someone turns to the Lord, the veil is removed. The Lord is the Spirit, and where the Spirit of the Lord is, there is freedom. And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another, for this comes from the Lord who is the Spirit.