J.I. Packer explains the difference between regeneration and sanctification. Packer’s writing is dense but he can say what most theologians say in 10 pages in just a couple.
“The concept [of sanctification] 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.
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). . . .
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.
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).”
~ J. I. Packer’s Concise Theology: A Guide to Historic Christian Beliefs