What Is Sanctification?

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:

Past

Present

Future

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)