God calls every person to mirror and reflect His holy character: "As He who called you is holy, you also be holy in all your conduct, because it is written, 'Be holy, for I am holy'" (1 Peter 1:15-16). Our problem is that in ourselves we are not holy; we are unholy. Yet the Bible refers to us as "saints." The term saint means "one who is holy." Since holiness is not found in ourselves, we must be made holy. The One who works to make us holy, to conform us to the image of Christ, is the Holy Spirit. As the Third Person of the Trinity, the Holy Spirit is no more holy than the Father and the Son. Yet we do not speak of the Holy Father, the Holy Son, and the Holy Spirit. That the Spirit of God is called the Holy Spirit is not so much because of His person (which is indeed holy) but because of His work, to make us holy.
It is the special work of the Holy Spirit to make us saints. He consecrates us. The Holy Spirit fulfills the role of the sanctifier. To be sanctified is to be made holy, or righteous. Sanctification is a process that begins the moment we become Christians. The process continues until death when the believer is made finally, fully, and forevermore righteous.
The Reformed faith is distinctive in its emphasis on the working of the Holy Spirit alone in regeneration. We do not assist the Holy Spirit in our rebirth. We reject outright any notion of cooperative effort in the rebirth of the believer. Sanctification, however, is a different matter. Our sanctification is a cooperative venture. We must work with the Holy Spirit to grow in sanctification. The apostle Paul expressed this idea in his letter to the church at Phillipi:
Therefore, my beloved, as you have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling; for it is God who works in you both to will and to do for His good pleasure. (Philippians 2:12-13)
The call to cooperation is one that involves work. We are to work in earnest. To work with fear and trembling does not suggest a spirit of terror but of reverence coupled with effort. We are consoled by the knowledge that we are not left to do this work alone or by our own efforts. God is working within us to accomplish our sanctification.
The Holy Spirit indwells the believer, working to bring about a more righteous life and heart. We must be careful, however, not to confuse the indwelling Spirit with any deification of the individual. The Spirit is in the believer and works with the believer, but does not become the believer. The Spirit works to produce sanctified human beings, not deified creatures. When the Spirit indwells us, He does not become human and we do not become gods. The Holy Spirit does not destroy our personal identities as human beings. In our sanctification we are to become godlike in character, but not in being.