Here are two good answers.
First, the Westminster Shorter Catechism (question 35) answers that sanctification is:
the work of God’s free grace [2 Thess. 2:13] whereby we
- are renewed in the whole man after the image of God [Eph. 4:23-24], and
- are enabled more and more to die unto sin, and live unto righteousness [Rom. 6:4, 6; 8:1]
Anthony A. Hoekema (1913-1988), in Saved by Grace, defines it as follows:
That gracious operation of the Holy Spirit,
involving our responsible participation,
by which he
- delivers us from the pollution of sin,
- renews our entire nature according to the image of God, and
- enables us to live lives that are pleasing to him.
And here is a helpful chart from Andy Naselli’s helpful Let Go and Let God? showing a bit of the diversity of the way the concept is understood in the NT:
|Initial sanctification (occurs simultaneously with justification and regeneration)||Progressive sanctification||Perfect, complete, or final sanctification (i.e., glorification)|
|“I am (or have been) sanctified.”||“I am being sanctified.”||“I will be sanctified.”|
|Sets a believer apart positionally from sin’s penalty and/or experientially from his “old man” in Adam (Rom. 6; Acts 20:32; 26:18; 1 Cor. 1:2; 6:11; Heb. 10:10, 14)||Sets a believer apart from sin’s power and practice (John 17:17; 2 Cor. 3:18; 7:1; Phil. 1:6)||Sets a believer apart from sin’s presence and possibility (Rom. 8:29-30; Phil. 3:21; 1 Thess. 3:12-13; Jude 24)|