How does sanctification relate to being a Christian?

Sanctification, as understood in historic Reformed theology, refers to the ongoing process of spiritual growth and holiness that occurs in the life of the believer after justification. It is a result of the work of the Holy Spirit and signifies that, though believers are declared righteous in Christ, they are also called to live in obedience to God's commands. As Paul points out, 'and such were some of you: but ye are washed, but ye are sanctified, but ye are justified in the name of the Lord Jesus' (1 Corinthians 6:11). This transformative process is vital because it reflects the believer's identity in Christ and the evidence of the Spirit’s work in their lives, leading to greater conformity to Christ's image.