Set apart for God
To sanctify means to make holy or set apart for God’s purposes. In the Bible, holiness is not mainly about looking more religious than other people. It means belonging to the God who is pure, good, and faithful.
Jesus prayed that his followers would be sanctified through God’s truth (John 17:17). Paul also describes believers as chosen for salvation through the Spirit’s sanctifying work (2 Thessalonians 2:13). So sanctification starts with God’s grace. He claims people as his own before they have managed to sort themselves out.
For this is the will of God, even your sanctification, that ye should abstain from fornication:
Rooted in Jesus’ sacrifice
Sanctification is not a second way of earning acceptance with God. Jesus’ death is the foundation. Hebrews says that believers are sanctified through the offering of Christ’s body, and that his one sacrifice has secured what was needed for those being sanctified (Hebrews 10:10, Hebrews 10:14).
That gives this subject both confidence and humility. Christians don’t grow in holiness to make God reluctantly love them. They grow because, in Christ, they have already been welcomed by him.
For by one offering he hath perfected for ever them that are sanctified.
A lifelong change
Sanctification also describes the ongoing change of a Christian’s life. Old patterns do not vanish overnight. Paul tells the Roman believers to offer themselves to righteousness, which leads towards holiness, and he connects their freedom from sin with the fruit of holiness (Romans 6:19, Romans 6:22).
This growth can look very ordinary. It may mean telling the truth when lying would be easier, forgiving someone who has hurt you, resisting bitterness, caring for a neighbour, or choosing prayer when anxiety rises. There will be failures. Still, God is patient, and his purpose is real.
God works, and we respond
The Bible holds two truths together. Believers are called to take their faith seriously, while God is the one giving them the desire and strength to do what pleases him (Philippians 2:12-13). It is not passive, as though growth happens without any choices. Nor is it lonely self-improvement.
God uses Scripture, prayer, repentance, Christian community, suffering, service, and daily obedience to form his people. Peter calls believers to reflect the character of the Holy One who called them (1 Peter 1:15-16). The aim is not perfectionism or pretending. It is a gradually transformed life that points towards Jesus.
For it is God which worketh in you both to will and to do of his good pleasure.