Jamieson-Fausset-Brown Bible Commentary
17. the Lord—Christ (2Co 3:14, 16; 2Co 4:5).
is that Spirit—is THE Spirit, namely, that Spirit spoken of in 2Co 3:6, and here resumed after the parenthesis (2Co 3:7-16): Christ is the Spirit and "end" of the Old Testament, who giveth life to it, whereas "the letter killeth" (1Co 15:45; Re 19:10, end).
where the Spirit of the Lord is—in a man's "heart" (2Co 3:15; Ro 8:9, 10).
there is liberty—(Joh 8:36). "There," and there only. Such cease to be slaves to the letter, which they were while the veil was on their heart. They are free to serve God in the Spirit, and rejoice in Christ Jesus (Php 3:3): they have no longer the spirit of bondage, but of free sonship (Ro 8:15; Ga 4:7). "Liberty" is opposed to the letter (of the legal ordinances), and to the veil, the badge of slavery: also to the fear which the Israelites felt in beholding Moses' glory unveiled (Ex 34:30; 1Jo 4:18).