…The word "Lord" in verses 17 and 18 always means Christ. He himself is the Spirit; as the close of verse 18 repeats: "this comes from the Lord who is the Spirit." Other features of this conception in Paul are found in 1 Corinthians 6:17: "he who is united to the Lord becomes one Spirit with him," and in Romans 8:911, where the divine principle which dwells in the faithful alternately is called the Spirit, the Spirit of God, the Spirit of Christ, and Christ.
Hendrikus Berkhof, (Richmond, VA: John Knox Press, 1964), pp. 2425; see also pp. 18, 2527