How do we know that sanctification is necessary for believers?

Sanctification is critical for every believer because it signifies the ongoing work of the Holy Spirit to make us more like Christ. In Philippians 4:6, we are encouraged to present our requests to God, which includes asking for His aid in our sanctification. The process of sanctification involves the active pursuit of holiness and the struggle against sin, as we mourn over our past sins and seek to be delivered from them. Romans 8:29 also underscores that we are predestined to be conformed to the image of Christ, highlighting the importance of sanctification in fulfilling God's purpose for us.