Sanctification is the spiritual process of us sinners becoming holy by the work of God as we are in union with Christ. When we are first regenerated and born again (second birth) we receive justification and initial definite sanctification. But as we live our Christian lives, we engage in progressive sanctification whereby we are made righteous as we die to our Old Man and put on our new identity in Christ. Sanctification is a lifelong process, it doesn’t end until our mortal lives end and we are glorified.
Therefore gird up the loins of your mind, be sober, and rest your hope fully upon the grace that is to be brought to you at the revelation of Jesus Christ; as obedient children, not conforming yourselves to the former lusts, as in your ignorance; but as He who called you is holy, you also be holy in all your conduct, because it is written, “Be holy, for I am holy.”1 Peter 1:13-16
“Be holy, for I am holy.” – Here the apostle Peter is quoting Leviticus 19:2. One of the essential and eternal and immutable attributes of God is that He is holy. He wishes for us, His children through adoption by union with Christ, to also be holy.
This process of sanctification is accomplished through the work of the Holy Spirit in us. He delivers us, as justified sinners, from the effects of sin. This process is synergistic in the sense that it requires our active participation, however, this process is both initiated and sustained by God so in that sense it is monergistic. So just as our salvation is monergistic, so is our sanctification. God creates the good works for us to walk in, and God gives us the strength and ability to mortify sin and to perform faith working by love.
“Sanctify them through thy truth: thy word is truth.”John 17:17
Here is John Calvin’s commentary on this verse.
17. Sanctify them by thy truth. This sanctification includes the kingdom of God and his righteousness; that is, when God renews us by his Spirit, and confirms in us the grace of renewal, and continues it to the end. He asks, first, therefore, that the Father would sanctify the disciples, or, in other words, that he would consecrate them entirely to himself, and defend them as his sacred inheritance. Next, he points out the means of sanctification, and not without reason; for there are fanatics who indulge in much useless prattle about sanctification, but who neglect the truth of God, by which he consecrates us to himself. Again, as there are others who chatter quite as foolishly about the truth and yet disregard the word, Christ expressly says that the truth, by which God sanctifies his sons, is not to be found any where else than in the word. Thy word is truth; for the word here denotes the doctrine of the Gospel, which the apostles had already heard from the mouth of their Master, and which they were afterwards to preach to others. In this sense Paul says that the Church has been cleansed with the washing of water by the word of life, (Ephesians 5:26) True, it is God alone who sanctifies; but as the Gospel is the power of God to salvation to every one that believeth, (Romans 1:16) whoever departs from the Gospel as the means must become more and more filthy and polluted. The truth is here taken, by way of eminence, for the light of heavenly wisdom, in which God manifests himself to us, that he may conform us to his image. The outward preaching of the word, it is true, does not of itself accomplish this, For that preaching is wickedly profaned by the reprobate; but let us remember that Christ speaks of the elect whom the Holy Spirit efficaciously regenerates by the word. Now, as the apostles were not altogether destitute of this grace, we ought to infer from Christ’s words, that sanctification is not instantly completed in us on the first day, but that we make progress in it through the whole course of our life, till at length God, having taken away from us the garment of the flesh, fills us with his righteousness.
We are all born with a corrupted sinful nature due to the consequences of the Fall which resulted in every person being born tainted by that original sin of Adam. This corruption is manifested in the guilt of sin and the pollution of sin in our lives. When we are regenerated and justified this removes the guilt of original sin, however, to remove the pollution and to restore the image of God in us is a lifelong endeavor. It also should be noted that sanctification is worked out in a private way as we wrestle with sin corrupting our nature, and seek to undo that pollution, but also we are intended to be sanctified as a corporate endeavor of the Body of Christ. This is very comforting. We are not alone in our battle against sin. The Holy Spirit indwells us and gives us the power to overcome sin and be victorious in Christ. So this renewal restores to us what was lost.
It should be noted that the Wesleyan doctrine of entire sanctification or sinless perfectionism is utterly unbiblical and must be rejected. Scripture is clear that if we say we have no sin the truth is not in us. It is impossible for us to refrain from all willful sin, let alone the sins we commit that we didn’t directly will. It puts a believer in a very dangerous position whereby they feel complacent, that they have achieved complete sanctification and don’t have to any longer struggle and battle against sin. Such a doctrine is Satanic! We Reformed reject this view.