Sanctification says the Westminster Shorter Catechism (Q.35), is "the work of God's free grace, whereby we are renewed in the whole man after the image of God, and are enabled more and more to die unto sin, and live unto righteousness." The concept is not of sin being totally eradicated (that is to claim too much) or merely counteracted (that is to say too little), but of a divinely wrought character change freeing us from sinful habits and forming in us Christlike affections, dispositions, and virtues. - J.I. Packer, Concise Theology
Unlike regeneration, there is much Spirit-filled human effort involved in sanctification. On the one hand, "it is God who is at work in you, both to will and to work for His good pleasure" (Phil. 2:13). "On the other hand," we're commanded in 1 Tim. 4:7, "discipline yourself for the purposes of godliness." God uses means of grace to sanctify us, chief of which are the personal and corporate spiritual disciplines. In the personal realm, these include intake of God's Word, prayer, private worship, fasting, silence and solitude, etc. These are balanced by disciplines we practice with the church: public worship, hearing God's Word preached, observance of the ordinances, corporate prayer, fellowship, etc. And all along, our confidence is not in ourselves, but in God. As Paul put it, "For I am confident of this very thing, that He who began a good work in you will perfect it until the day of Christ Jesus" (Phil. 1:6). - Donald S. Whitney
In addition to the initial work of conviction, repentance, and regeneration, the Spirit provides for the believer's ongoing sanctification.
1. He indwells believers
At the moment of salvation the Holy Spirit takes up residence in the believer's heart.
a. Acts 2:38 -- Peter said, "Repent, and let each of you be baptized in the name of Jesus Christ for the forgiveness of your sins; and you shall receive the gift of the Holy Spirit."
b. 1 Corinthians 12:13 -- Paul said, "By one Spirit we were all baptized into one body... we were all made to drink of one Spirit."
c. Romans 8:9 Paul said, "You are not in the flesh but in the Spirit.... If anyone does not have the Spirit of Christ, he does not belong to Him."
d. 1 Corinthians 6:19 -- Paul said, "Your body is a temple of the Holy Spirit who is in you, whom you have from God."
2. He baptizes believers into the Body of Christ
The "Body of Christ" is a metaphor Paul used for the true church. Christ is its Head and individual believers are its various parts. Believers are placed into the Body of Christ at the moment of their salvation (1 Cor. 12:13).
3. He gives spiritual gifts to believers
Spiritual gifts are divine enablements for spiritual service such as preaching, teaching, governing, faith, helps, and many others. Each believer has a unique combination of gifts specially suited to him or her by the Holy Spirit. In that sense believers are like spiritual snowflakes no two are alike. Each has a unique and important role to play within the church.
4. He secures every believer's eternal inheritance
Ephesians 1:13 14 says, "You were sealed in Him with the Holy Spirit of promise, who is given as a pledge of our inheritance." The Greek word translated "sealed" speaks of authenticity an unbreakable promise. The Holy Spirit secures you for eternity.
The Greek word translated "pledge" (arrabon) speaks of a down payment that was made to secure a purchase. The moment you were saved, the Spirit of God became the down payment on God's final installment of your eternal glory. In modern Greek, arrabon [arrabon] includes the idea of an engagement ring. In that sense we can view the Holy Spirit as God's engagement ring a sign of His deep love and His guarantee to keep His promises.
5. He sanctifies believers
Second Thessalonians 2:13 says, "God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth." In that context "sanctification" means we have been set apart from sin and eternal death. That's a positional sanctification resulting from our standing as believers in Christ. Our practical sanctification comes as the Spirit does His transforming work within us day by day (2 Cor. 3:18).
The Christian life begins in the power of the Spirit and includes all those aspects of His supernatural work within us. That's why we should never attempt to reach spiritual maturity by human means apart from His power. Instead, we must understand and draw from the ongoing resources He provides for our sanctification.