One does not have to read far in Romans—the most systematic of all of Paul’s letters—to encounter a reference to the Spirit’s sanctifying work. In Romans 1:4 Paul describes the Spirit with a phrase that is unique in the New Testament—he is the “Spirit of holiness.” What exactly does the Apostle mean by describing the Spirit thus? Why does he not use the more common term “Holy Spirit”? For some writers the terms “Holy Spirit” and “Spirit of holiness” are simply synonymous and they would understand the term “Spirit of holiness” to mean something like “the Spirit whose character is holiness.” There is another way, though, to understand this phrase and that is to see it as a description of the Spirit’s work: he is the giver of holiness, the One who supplies holiness to all who call upon the name of Jesus. Given the Old Testament form of the phrase “Spirit of holiness,” the latter interpretation is probably the better of the two. It highlights the fact that central among the activities of the Spirit is the sanctification of the people of God. In fact, for Paul as for the other New Testament authors, the Holy Spirit is indispensable for living a life that pleases God.
Another key text with regard to the Spirit’s sanctifying work is found in Romans 15:8-21. Here, the Apostle begins by indicating that one of the ultimate goals of Christ’s ministry was that Gentiles might come to glorify the God of Israel for being a God of mercy. The citation of four Old Testament texts, drawn from various parts of the Old Testament canon, supports this affirmation (Romans 15:8-12). Christ’s intentions with regard to the Gentiles is of central concern to the Apostle for he has been called by God to preach Christ among the Gentiles where the name of Jesus has never been heard (Romans 15:20), or, as he puts it, “to be a minister of Christ Jesus to the Gentiles in the priestly service of the gospel of God” (Romans 15:16). Using imagery drawn from the Temple worship of Israel to describe his ministry, Paul argues that Gentiles—who were formerly ritually impure and thus utterly unacceptable to God—have now become acceptable to God. In the immediate context of these verses, what has made them acceptable is their embrace of the gospel, which, in turn, was made possible by the Holy Spirit’s power (Romans 15:19). In Paul’s words, they have been “sanctified by the Holy Spirit” (Romans 15:16), that is, set apart to serve God and to fulfill his purposes, which, because of God’s holy character, inevitably involves leading lives of godliness. It is on the basis of this sanctifying work of the Spirit that Paul, later in this chapter and in the one that follows, can call believers “saints” (Romans 15:25-26, 31; 16:1, 15).
Earlier in this letter, the sanctifying work of the Spirit had also been highlighted in Romans 8:1-4. Christ came into the world so that those who believe in him would be able to truly obey the essence of the Law (Romans 8:4). Central to Christ’s death is the liberation of men and women from the death-dealing bondage of sin. This obedience and freedom is made a reality in believers by the Spirit, who is none other than the “Spirit of life,” that is the Spirit of the living God, the source of all that is good. Thus, the liberating work of the Spirit is rooted in the saving work of Christ (Romans 8:2).
Again in this chapter, Paul emphasizes that the Spirit’s indwelling presence in the life of the believer provides him or her with rich resources to fight sin: Romans 8:12-14. Although the believer has been radically delivered from sin’s tyranny, this does not mean—as so much of the teaching of the New Testament makes clear—that he or she now experientially enjoys perfect holiness. There is an ongoing battle with sin and thus the necessity of heeding the Apostle’s admonition to mortify sin (Romans 8:13).
This work of mortification—the “gradual annihilation of all the remainders of this cursed life of sin,” as the Puritan author John Owen (1616-1683) aptly puts it—involves the believer’s complete involvement, though ultimately it is the Spirit’s work. Owen well sums up the Apostle’s thought in this regard when he states in his classic exposition of Romans 8:13, The Mortification of Sin in Believers (1656), that the Spirit
"doth not so work our mortification in us as not to keep it still an act of our obedience. The Holy Ghost works in us and upon us, as we are fit to be wrought in and upon; that is, so as to preserve our own liberty and free obedience. He works upon our understandings, wills, consciences, and affections, agreeably to their own natures; he works in us and with us, not against us or without us; so that his assistance is an encouragement as to the facilitating of the work, and no occasion of neglect as to the work itself.”
In other words, this is a variation on one of the central ethical principles of the New Testament: be what you are. Because you are saints lead holy lives; live in holy conformity with the Spirit who indwells you. Since he is holy, be holy. Paul puts it this way at the close of another well-known passage that deals with the sanctifying work of the Spirit: “if we live by the Spirit, let us also walk by the Spirit” (Galatians 5:25, ESV).
 As James D.G. Dunn notes, the term “Spirit of holiness” would almost certainly be understood by Paul and the first Christians as denoting the Holy Spirit” [Romans 1-8 (Word Biblical Commentary, vol.38A; Dallas: Word, 1988), 14-15. See also Thomas R. Schreiner, Romans (Grand Rapids: Baker, 1998), 43.
 C.E.B. Cranfield, Romans: A Shorter Commentary (Grand Rapids: William B. Eerdmans Publ. Co., 1985), 7; Gordon D. Fee, God’s Empowering Presence: The Holy Spirit in the Letters of Paul (Peabody, Massachusetts: Hendrickson Publishers, 1994), 483.
 See Smith, “Pauline Studies: Pauline Pneumatology.”
 See the similar idea in 1 Corinthians 6:11. See also the comments of James D.G. Dunn, Romans 9-16 (Word Biblical Commentary, vol.38B; Dallas: Word, 1988), 860-861; Fee, God’s Empowering Presence, 626-627; David Peterson, Possessed by God. A New Testament Theology of Sanctification and Holiness (Grand Rapids: William B. Eerdmans Publ. Co., 1995), 58-59; Schreiner, Romans, 766-767.
 Cranfield, Romans, 174; Fee, God’s Empowering Presence, 519-538.
 A Discourse Concerning the Holy Spirit [The Works of John Owen (1850-1853 ed., 16 volumes; repr. London: The Banner of Truth Trust, 1965-1968), III, 545].
 Of the Mortification of Sin in Believers (Works, VI, 20). See also the comments of J. I. Packer, “ ‘Keswick’ and the Reformed Doctrine of Sanctification”, The Evangelical Quarterly, 27 (1955), 156.