“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.” (WSC Q/A 35). This definition of sanctification is a good summary of Scripture’s teaching (Ezek. 36:37, Phil. 2:13, 2 Thes. 2:13, Eph 4>23-24, Rom 6:4-14, etc.). Thomas Watson does a nice job giving some more detail:
- Sanctification is a supernatural thing. It is divinely infused. We are naturally polluted, and to cleanse, God takes it to be his gracious prerogative. “I am the Lord which sanctify you” (Lev. 21:8). Sanctification is a flower of the Spirit’s planting, therefore it is called, ‘The sanctification of the Spirit’ (1 Pet. 1:2).
- Sanctification is an intrinsic thing. It lies chiefly in the heart. It is called “the adorning the hidden man of the heart” (1 Pet. 3:4). The dew wets the leaf, the sap is hid in the root… sanctification is deeply rooted in the soul (Ps. 51:6).
- Sanctification is an extensive thing: it spreads to the whole man. “The God of peace sanctify you wholly” (1 Thes. 5:23). As original corruption has depraved all the faculties, the heart, mind, and will, sanctification is the renewing of all the faculties. Therefore in Scripture it is called a “new man,” not a new eye or a new tongue, but a “new man” (Col. 3:10). A Christian, though he be sanctified only in part, yet he is sanctified in every part.
- Sanctification is an intense and ardent thing. Its properties burn within the believer: “fervent in spirit” (Rom. 12:11). Sanctification is enflamed into zeal. Sanctification means a person’s religion is heated to some degree, and his heart boils over in love to God.
- Sanctification is an abiding thing. “His seed remains in him” is how 1 John 3:9 puts it. He who is truly sanctified cannot fall from that state. Indeed, sanctification may suffer an eclipse (Rev. 2:4), but sanctification will abide: “The anointing which you have received abides in you” (1 John 2:27).
- Sanctification is a progressive thing. It is growing; it is compared to a seed which grows (1 Pet. 1:23, 1 John 3:9). Justification does not have degrees; a believer cannot be more justified or elected than he is, but he may be more sanctified than he is. Sanctification is still increasing, like the morning sun, which grows brighter to full noon.
Watson’s helpful notes (which I’ve edited and shortened slightly) are on page 242 of his excellent book, A Body of Divinity (a commentary on the Shorter Catechism).