Now may the God of peace himself sanctify you completely, and may your whole spirit and soul and body be kept blameless at the coming of our Lord Jesus Christ. He who calls you is faithful; he will surely do it. (1 Thessalonians 5:23-24 ESV)
In studying this passage, I have been helped by the insights of Herman Bavinck on the topic of sanctification. Some highlights below...
"To understand the benefit of sanctification correctly, we must proceed from the idea that Christ is our holiness in the same sense in which he is our righteousness. He is a complete and all-sufficient Savior. He does not rest until, after pronouncing his acquittal in our conscience, he has also imparted full holiness and glory to us. By his righteousness, accordingly, he does not just restore us to the state of the just who will go scot-free in the judgment of God, in order then to leave us to ourselves to reform ourselves after God's image and to merit eternal life. But Christ has accomplished everything. He bore for us the guilt and punishment of sin, placed himself under the law to secure eternal life for us, and then arose from the grave to communicate himself to us in all his fullness for both our righteousness and sanctification (1 Cor 1:30). The holiness that must completely become ours therefore fully awaits us in Christ." (Reformed Dogmatics, 4:248)
"The new life in Christ, just like all natural life, must be nourished and strengthened. This is possible only in communion with Christ in the Holy Spirit and through the word of Scripture. Enlightened by the Spirit, believers gain a new knowledge of faith. The gospel is the food of faith and must be known to be nourishment. Salvation that is not known and enjoyed is no salvation. God saves by causing himself to be known and enjoyed in Christ." (RD 4:96)
"Faith is not intellectual assent to a historical truth but a practical knowledge of the grace that God has revealed in Christ, a heartfelt trust that he has forgiven all our sins and accepted us as his children. For that reason this faith is not only needed at the beginning in justification, but it must also accompany the Christian throughout one's entire life, and also play a permanent and irreplaceable role in sanctification. In sanctification, too, it is exclusively faith that saves us. . . Faith . . . is the one great work Christians have to do in sanctification according to the principles of the gospel (John 6:29); it is the means of sanctification par excellence. . . . Faith breaks all self-reliance and fastens on to God's promise. It allows the law to stand in all its grandeur and refuses to lower the moral ideal, but also refrains from any attempt, by observing it, to find life and peace; it seizes upon God's mercy and relies on the righteousness and holiness accomplished in Christ on behalf of humans. It fosters humility, dependence, and trust and grants comfort, peace, and joy through the Holy Spirit." (RD 4:257)
“The life of the Christian is not a quiet growth, but a continuous struggle... Because sanctification, like the whole of salvation, is the work of God, we are admonished, obliged, to a new obedience, and we are also qualified for it. He grants abundant grace not that we should instantly or suddenly be holy and continue to rest in this holiness, but that we should persevere in the struggle and remain standing. He hears our prayers but does it in accordance with the law and order which He has fixed for the spiritual life. Hence we are always of good course, for He who has begun a good work in us will finish it until the day of Jesus Christ. The believers can and they will become holy because in Christ they are holy.” (Our Reasonable Faith, pp 492, 502-3).