By J.I. Packer (from Concise Theology)
"Do you not know that the wicked will not inherit the kingdom of God?...And that is what some of you were. But you were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God" (1 Cor. 6:9, 11).
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.
Sanctification is an ongoing transformation within a maintained consecration, and it engenders real righteousness within the frame of relational holiness. Relational sanctification, the state of being permanently set apart for God, flows from the cross, where God through Christ purchased and claimed us for himself (Acts 20:28; 26:18; Heb. 10:10). Moral renovation, whereby 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 sanctity 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 the hallowing and glorifying of God's name in this world; desire to pray, worship, love, serve, honor, and please God; desire to show love and bring benefit to others. In sanctification, the Holy Spirit "works in you to will and to act" according to God's purpose; what he does is prompt you to "work out your salvation" (i.e., express it in action) by fulfilling these new 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). Paul's use of glory in 2 Corinthians 3:18 shows that for him sanctification of character is glorification begun. Then the physical transformation that gives us a body like Christ's, one that will match our totally transformed character and be a perfect means of expressing it, will be glorification completed (Phil. 3:20-21; 1 Cor. 15:49-53).
Regeneration was a momentary monergistic act of quickening the spiritually dead. As such, it was God's work alone. Sanctification, however, is in one sense synergistic - it is an ongoing cooperative process in which regenerate persons, alive to God and freed from sin's dominion (Rom. 6:11, 14-18), are required to exert themselves in sustained obedience. God's method of sanctification is neither activism (self-reliant activity) nor apathy (God-reliant passivity), but God-dependent effort (2 Cor. 7:1; Phil. 3:10-14; Heb. 12:14). Knowing that without Christ's enabling we can do nothing, morally speaking, as we should, and that he is ready to strengthen us for all that we have to do (Phil. 4:13), we "stay put" (remain, 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).
The standard to which God's work of sanctifying his saints is directed is his own revealed moral law, as expounded and modeled by Christ himself. Christ's love, humility, and patience under pressure are to be consciously imitated (Eph. 5:2; Phil. 2:5-11; 1 Pet. 2:21), for a Christlike spirit and attitude are part of what law-keeping involves.
Believers find within themselves contrary urgings. The Spirit sustains their regenerate desires and purposes; their fallen, Adamic instincts (the "flesh") which, though dethroned, are not yet destroyed, constantly distract them from doing God's will and allure them along paths that lead to death (Gal. 5:16-17; James 1:14-15). To clarify the relationship between the law and sin, Paul analyzes in a personal and dramatic way the sense of impotence for complete law-keeping, and the enslavement to behavior one dislikes, that the Spirit-flesh tension produces (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 "mortify" (i.e., drain the life out of, weaken as a means of killing) particular bad habits, and in that sense more and more die unto sin (Rom. 8:13; Col. 3:5). They will experience many particular deliverances and victories in their unending battle with sin, while never being exposed to temptations that are impossible to resist (1 Cor. 10:13).