What is Sanctification? by Nathan W. Bingham (original source “What is sanctification?” The Reformation Study Bible’s theological article on “Sanctification” provides a clear and concise answer.

According to the Westminster Shorter Catechism (Q. 35), sanctification 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.” It is a continuing change worked by God in us, freeing us from sinful habits and forming in us Christlike affections, dispositions, and virtues. It does not mean that sin is instantly eradicated, but it is also more than a counteraction, in which sin is merely restrained or repressed without being progressively destroyed. Sanctification is a real transformation, not just the appearance of one.

GOD CALLS HIS CHILDREN TO HOLINESS, AND GRACIOUSLY GIVES WHAT HE COMMANDS.
The basic meaning of “sanctify” is to set apart to God, for His use. But God works in those whom He claims as His own to conform them “to the image of his Son” (Rom. 8:29). This moral renovation, in which we are increasingly changed from what we once were, flows from the agency of the indwelling Holy Spirit (Rom. 8:13; 12:1, 2; 1 Cor. 6:11, 19, 20; 2 Cor. 3:18; Eph. 4:22–24; 1 Thess. 5:23; 2 Thess. 2:13; Heb. 13:20, 21). God calls His children to holiness, and graciously gives what He commands (1 Thess. 4:4; 5:23).

Regeneration is birth; sanctification is growth. In regeneration, God implants desires that were not there before: desire for God, for holiness, and for glorifying God’s name in the world; desire to pray and worship; desire to love and bring benefit to others. In sanctification, the Holy Spirit “works in you, both to will and to work” according to God’s purpose, enabling His people to fulfill their new, godly desires (Phil. 2:12, 13). Christians become increasingly Christlike, as the moral profile of Jesus (the “fruit of the Spirit”) is progressively formed in them (2 Cor. 3:18; Gal. 4:19; 5:22–25).

Regeneration is a momentary act, bringing a person from spiritual death to life. It is exclusively God’s work. Sanctification is an ongoing process, dependent on God’s continuing action in the believer, and consisting of the believer’s continuous struggle against sin. God’s method of sanctification is neither activism (self-reliant activity) nor apathy (God-reliant passivity), but human effort dependent on God (2 Cor. 7:1; Phil. 3:10–14; Heb. 12:14). Knowing that without Christ’s enabling we cannot do good works, but also that He is ready to strengthen us for all we have to do (Phil. 4:13), we “abide” in Christ, asking for His help constantly— and we receive it (Col. 1:11; 1 Tim. 1:12; 2 Tim. 1:7; 2:1).

REGENERATION IS BIRTH; SANCTIFICATION IS GROWTH.
The standard to which God’s work of sanctifying His saints is directed is His own revealed moral law, expounded and modeled by Christ Himself. Christ’s love, humility, and patience are a supreme standard for Christians (Rom. 13:10; Eph. 5:2; Phil. 2:5–11; 1 Pet. 2:21).

Believers find within themselves contrary urgings. The Spirit sustains their regenerate desires and purposes, but their fallen instincts (the “flesh”) obstruct their path and drag them back. The conflict of these two is sharp. Paul says he is unable to do what is right, and unable to restrain himself from doing what is wrong (Rom. 7:14–25). This conflict and frustration will be with Christians as long as they are in the body. Yet by watching and praying against temptation, and cultivating opposite virtues, they may through the Spirit’s help “put to death” particular bad habits (Rom. 8:13; Col. 3:5). They will experience many particular deliverances and victories in their battle with sin, while not being exposed to temptations that are impossible to resist (1 Cor. 10:13).