Answer: Sanctification is a work of God’s grace, whereby they whom God has, before the foundation of the world, chosen to be holy, are in time, through the powerful operation of his Spirit₁ applying the death and resurrection of Christ unto them₂, renewed in their whole man after the image of God₃; having the seeds of repentance unto life, and all other saving graces, put into their hearts₄, and those graces so stirred up, increased, and strengthened₅, as that they more and more die unto sin, and rise unto newness of life.₆

1. Ephesians 1:4; 1 Corinthians 6:11; 2 Thessalonians 2:13
2. Romans 6:4-6;
3. Ephesians 4:23-24
4. Acts 11:18; 1 John 3:9
5. Jude 1:20; Hebrews 6:11-12; Ephesians 3:16-19; Colossians 1:10-11
6. Romans 6:4; Romans 6:6; Romans 6:14; Galatians 5:24

For further study see:

1. “The Two Sides of Sanctification”, by Dr. Jeff Landis, Covenant Orthodox Presbyterian Church, San Jose.
   
2. WSC 36: What are the benefits which in this life do accompany or flow from justification, adoption, and sanctification?
3. WLC 77: Wherein do justification and sanctification differ?
4. WLC 78: Whence arises the imperfection of sanctification in believers
5. WCF CHAPTER 13: Of Sanctification