The Westminster Shorter Catechism answers the question “What is sanctification?” by stating:
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.
This is an excellent, short explanation of what sanctification is. The London Baptist Confession of 1689, which is modeled after the Westminster Confession, elaborates on this important doctrine.
In regards to salvation, Chapter 13 of the 1689 London Baptist Confession states,
1. They who are united to Christ, effectually called, and regenerated, having a new heart and a new spirit created in them through the virtue of Christ’s death and resurrection, are also farther sanctified, really and personally, through the same virtue, by His Word and Spirit dwelling in them; the dominion of the whole body of sin is destroyed, and the several lusts thereof are more and more weakened and mortified, and they more and more quickened and strengthened in all saving graces, to the practice of all true holiness, without which no man shall see the Lord.
Applicable Verses: Acts 20:32; Romans 6:5, 6; John 17:17; Ephesians 3:16-19; 1 Thessalonians 5:21-23; Romans 6:14; Galatians 5:24; Colossians 1:11; 2 Corinthians 7:1; Hebrews 12:14
2. This sanctification is throughout the whole man, yet imperfect in this life; there abideth still some remnants of corruption in every part, whence ariseth a continual and irreconcilable war; the flesh lusting against the Spirit, and the Spirit against the flesh.
Applicable Verses: 1 Thessalonians 5:23; Romans 7:18, 23; Galatians 5:17; 1 Peter 2:11
3. In which war, although the remaining corruption for a time may much prevail, yet through the continual supply of strength from the sanctifying Spirit of Christ, the regenerate part doth overcome; and so the saints grow in grace, perfecting holiness in the fear of God, pressing after an heavenly life, in evangelical obedience to all the commands which Christ as Head and King, in His Word hath prescribed them.
Applicable Verses: Romans 7:23; Romans 6:14; Ephesians 4:15, 16; 2 Corinthians 3:18; 2 Corinthians 7:1
For more on the doctrine of sanctification, check out this episode of the Reasonable Theology podcast: