2Cor 3:6-8
"who has made us sufficient to be ministers of a new covenant, not of the letter but of the Spirit. For the letter kills, but the Spirit gives life."
The antithesis letter/Spirit is not OT/NT in general — it is law as external demand vs. Spirit as internal transformation. The law kills because it reveals sin without providing power to obey; the Spirit gives life because it transforms from within.
2Cor 3:17-18
"Now 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."
The progressive transformation: metamorphoumetha apo doxes eis doxan — from glory to glory. The agent is the Spirit. The contemplation of Christ transforms — sanctification is essentially contemplative.