Sanctification is the ongoing process by which God makes believers holy — conforming them to the image of Christ through the work of the Holy Spirit. It is distinct from justification (being declared righteous at the moment of faith) and glorification (being made perfectly holy at Christ's return). Sanctification is both God's work and a human responsibility, both already accomplished in Christ and not yet fully realized. 98/100 Christ-Centered.
Understanding sanctification requires understanding where it fits in the overall structure of salvation. Scripture uses salvation language in three tenses:
Justification (past, completed): At the moment of genuine faith, God declares the believer righteous — not because they are righteous but because Christ's righteousness is credited to them (Romans 5:1, 2 Corinthians 5:21). This is instantaneous and complete. It cannot be added to or subtracted from.
Sanctification (present, ongoing): The lifelong process of becoming in practice what you already are in Christ. 2 Corinthians 3:18 — "We all, who with unveiled faces contemplate the Lord's glory, are being transformed into his image with ever-increasing glory, which comes from the Lord, who is the Spirit." This is progressive and incomplete in this life.
Glorification (future, certain): The final and complete making-holy that occurs at Christ's return or the believer's death. Philippians 1:6 — "He who began a good work in you will carry it on to completion until the day of Christ Jesus." 1 John 3:2 — "When Christ appears, we shall be like him, for we shall see him as he is."
Scripture speaks of sanctification in both a definitive (past, completed) and a progressive (present, ongoing) sense:
Definitive sanctification: At conversion, believers are set apart for God — positionally holy. 1 Corinthians 6:11 — "You were washed, you were sanctified, you were justified in the name of the Lord Jesus Christ and by the Spirit of our God." This is past tense — already accomplished.
Progressive sanctification: The ongoing transformation toward Christlikeness. Romans 12:2 — "Do not conform to the pattern of this world, but be transformed by the renewing of your mind." Philippians 2:12-13 — "Work out your salvation with fear and trembling, for it is God who works in you to will and to act in order to fulfill his good purpose." Both the human responsibility ("work out") and the divine empowerment ("it is God who works in you") are stated in the same breath.
The means of sanctification in Scripture: the Word of God (John 17:17 — "Sanctify them by the truth; your word is truth"), the Holy Spirit (1 Peter 1:2, 2 Thessalonians 2:13), prayer, suffering (Romans 5:3-4 — suffering produces perseverance, character, hope), and Christian community (Hebrews 10:24-25 — "spur one another on toward love and good deeds"). Sanctification is not a solo project — it happens in relationship with God and with God's people.
See our guide on Who Is the Holy Spirit? and our guide on What Is Justification by Faith? See our Theology hub. The Gospel Coalition's essay on sanctification provides thorough theological treatment. GotQuestions on sanctification is accessible and comprehensive.
For the foundational question of what makes someone a Christian, see our guide What Is a Christian?
For the question of who God predestines for salvation, see our guide What Is Predestination?