What is sanctification?
Sanctification is the work of God's free grace1, whereby we are renewed in the whole man after the image of God2, and are enabled more and more to die unto sin, and live unto righteousness3.
Scripture proofs
The Westminster Assembly's proof texts (KJV). The bracketed numbers in the answer above mark the clause each set of references supports; expand a row to read the verses.
1 Ezk. 36:27; Phil. 2:13; 2 Thess. 2:13
And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
For it is God which worketh in you both to will and to do of his good pleasure.
But we are bound to give thanks alway to God for you, brethren beloved of the Lord, because God hath from the beginning chosen you to salvation through sanctification of the Spirit and belief of the truth:
2 2 Cor. 5:17; Eph. 4:23-24; 1 Thess. 5:23
Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.
Eph. 4:23 And be renewed in the spirit of your mind;
Eph. 4:24 And that ye put on the new man, which after God is created in righteousness and true holiness.
And the very God of peace sanctify you wholly; and I pray God your whole spirit and soul and body be preserved blameless unto the coming of our Lord Jesus Christ.
3 Ezek. 36:25-27; Rom. 6:4, 6, 12-14; 2 Cor. 7:1; IPet. 2:24
Ezek. 36:25 Then will I sprinkle clean water upon you, and ye shall be clean: from all your filthiness, and from all your idols, will I cleanse you.
Ezek. 36:26 A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh.
Ezek. 36:27 And I will put my spirit within you, and cause you to walk in my statutes, and ye shall keep my judgments, and do them.
Rom. 6:4 Therefore we are buried with him by baptism into death: that like as Christ was raised up from the dead by the glory of the Father, even so we also should walk in newness of life.
Rom. 6:6 Knowing this, that our old man is crucified with him, that the body of sin might be destroyed, that henceforth we should not serve sin.
Rom. 6:12 Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.
Rom. 6:13 Neither yield ye your members as instruments of unrighteousness unto sin: but yield yourselves unto God, as those that are alive from the dead, and your members as instruments of righteousness unto God.
Rom. 6:14 For sin shall not have dominion over you: for ye are not under the law, but under grace.
Having therefore these promises, dearly beloved, let us cleanse ourselves from all filthiness of the flesh and spirit, perfecting holiness in the fear of God.
Who his own self bare our sins in his own body on the tree, that we, being dead to sins, should live unto righteousness: by whose stripes ye were healed.
This doctrine across the Standards
Where the Confession and the Westminster Larger Catechism treat the same matter, so the question can be studied alongside its parallel statements.
In the Westminster Confession
XIII.1 They who are effectually called and regenerated, having a new heart and a new spirit created in them, …
They who are effectually called and regenerated, having a new heart and a new spirit created in them, are further sanctified, really and personally, through the virtue of Christ's death and resurrection, 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 true holiness, without which no man shall see the Lord.
→ XIII.1 · Of SanctificationXIII.2 This sanctification is throughout in the whole man, yet imperfect in this life; there abideth still some remnants …
This sanctification is throughout in 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.
→ XIII.2 · Of SanctificationXIII.3 In which war, although the remaining corruption for a time may much prevail, yet, through the continual supply …
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.
→ XIII.3 · Of SanctificationIn the Westminster Larger Catechism
Q.75 What is sanctification?
Sanctification is a work of God's grace, whereby they whom God hath, before the foundation of the world, chosen to be holy, are in time, through the powerful operation of his Spirit applying the death and resurrection of Christ unto them, renewed in their whole man after the image of God; having the seeds of repentance unto life, and all other saving graces, put into their hearts, and those graces so stirred up, increased, and strengthened, as that they more and more die unto sin, and rise unto newness of life.
→ Open Q.75Q.77 Wherein do justification and sanctification differ?
Although sanctification be inseparably joined with justification, yet they differ, in that God in justification imputeth the righteousness of Christ; in sanctification his Spirit infuseth grace, and enableth to the exercise thereof; in the former, sin is pardoned; in the other, it is subdued: the one doth equally free all believers from the revenging wrath of God, and that perfectly in this life, that they never fall into condemnation; the other is neither equal in all, nor in this life perfect in any, but growing up to perfection.
→ Open Q.77Q.78 Whence ariseth the imperfection of sanctification in believers?
The imperfection of sanctification in believers ariseth from the remnants of sin abiding in every part of them, and the perpetual lustings of the flesh against the spirit; whereby they are often foiled with temptations, and fall into many sins, are hindered in all their spiritual services, and their best works are imperfect and defiled in the sight of God.
→ Open Q.78