How do we know that union with Christ is necessary for sanctification?

The concept of union with Christ is central to the doctrine of sanctification in Reformed theology. As believers are united with Christ in His crucifixion, they are empowered to live holy lives. This is illustrated in Paul's statement, "I am crucified with Christ; nevertheless I live; yet not I, but Christ lives in me" (Galatians 2:20). This union provides not only the grounds for forgiveness but also the means by which the believer can overcome sin and grow in holiness through the Spirit, illustrating the transformative essence of a relationship with a crucified and resurrected Lord.