…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 8:9-11, 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, The Doctrine of the Holy Spirit (Richmond, VA: John Knox Press, 1964), pp. 24-25; see also pp. 18, 25-27