What is Sanctification?
We are born sinners, unable to please God. The consequence of our first parent’s (Adam and Eve) sin is our own inability not to sin (Romans 3:10-12).
But “Thanks be to God, who delivers us through Jesus Christ our Lord (Romans 7:25)!” Jesus Christ, the unique God-man, is the sinner's substitute in both judgment and in righteousness. Those who repent (turn from sin as you cling in faith to Christ alone) are forgiven and declared righteous in Him (2 Corinthians 5:21; Ephesians 2:8). The Christian is then given a new nature that enables him/her to submit to God in obedience (Romans 8:7-12).
The moment a believer is declared righteous through faith in Christ, he/she is also empowered by the Holy Spirit to walk in righteousness. This isn't instant perfectionism but the gradual process of yielding to one’s new nature while denying the old sin nature (Ephesians 4:22-24). It is an active and ongoing conformity with Christ.
While justification is the work of God alone, sanctification is the work of both God and the believer. Nevertheless, we do not view the process as equal cooperation--half from God and half from the Christian. Derek Thomas writes: "True sanctification (our perseverance to the end) does not take place over our heads. But our holiness is achievable only because God works in us...without God's work in us, we cannot do anything."
God works in you that you might work out the fruit of your justification in obedience that pleases Him (Philippians 2:12-13). He uses various means of maturing the believer toward holiness, with the Word of God being the primary means of His work. Jesus prayed that His Father might sanctify believers by the truth of His Word (John 17:17).
The will of God for the Christian is sanctification (1 Thessalonians 4:3)! Know that a faithful church is a gift and a gracious means toward this end. There, the Gospel is proclaimed and you are joined with others for accountability, mutual encouragement, repentance and the practice of Christian disciplines. So link arms with other believers and pursue holiness knowing that the Lord will accomplish His perfect work in you--even your glorification (ultimate sanctification) (1 John 3:2-3; Jude 1:24)!
Please see the page with the Gospel for a fuller understanding of original sin, the uniqueness of Christ, justification and one's appropriate response to the Gospel.
1 Derek W. H. Thomas, How the Gospel Brings us all the Way Home (Orlando FL: Reformation Trust, 2011), 51.