Romans 1:4 uses an interesting expression for the Holy Spirit. There, the Spirit is called the Spirit of holiness.

What St. Paul seems to be doing by using that expression is proclaiming the Spirit’s sanctifying work in those who are in Christ. He is the Spirit of holiness— in other words, God’s Spirit who makes us holy.

When we talk about sanctification, most Christians have in mind primarily personal, progressive moral betterment.

But something much more astounding is actually taking place. Look at Romans 1:4 in its entirety:

… and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord
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ESV

The Spirit declared Christ the Son of God by raising Him from the dead.

Now, look at what Paul says about the work of the Spirit in the lives of believers just a few chapters later:

If the Spirit of him who raised Jesus from the dead dwells in you, he who raised Christ Jesus from the dead will also give life to your mortal bodies through his Spirit who dwells in you … For all who are led by the Spirit of God are sons of God. For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, “Abba! Father!” The Spirit himself bears witness with our spirit that we are children of God, and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him.
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Romans‬ ‭8:11, 14-17‬ ‭ESV‬

There are some direct parallels between what the Spirit does for Christ in Romans 1:4–raising Him from the dead and declaring Him to be God’s Son—and what the Spirit does for those who believe in Christ in Romans 8.

The Spirit raises us from death to life (Ephesians 2:1-5), and will resurrect us at the last day, when we will be declared children of God before all creation (Romans 8:21-23). In the meantime, the Spirit proclaims to our spirit that we are God’s sons and daughters, and co-heirs with Christ.

From the moment of our baptism, the Spirit reminds us that the Father says of us what He says of Christ: You are my beloved Son; with you I am well pleased. (Luke‬ ‭3:22‬ ‭ESV‬‬)

In this life, the Spirit transforms us into the image of Christ, from one degree of glory to another (2 Corinthians 3:18; cf. Romans 8:29).

And when Christ returns, the Spirit will perfect our transformation into Christ’s image by resurrecting us in the likeness of Christ’s glorified resurrection body (Philippians 3:21; 1 John 3:2).

But notice very carefully what Paul says: this comes from the Lord, who is the Spirit (2 Corinthians 3:18 ESV). Sanctification is the work of the Spirit. It’s not synergistic, where we do our part, and the Spirit does His part.

Our “part” is a daily dying and rising with Christ (Luke 9:23; Galatians 2:20; cf. Romans 6:4). God doesn’t kill our old Adam all at once.

Sanctification is a daily process of dying and rising again—a daily renewal and reenactment of our baptism—by which the Spirit of holiness bears and matures His good fruit in us (Galatians 5:22-23); and transforms us into the image of Christ, from one degree of glory to another.