How do we know the doctrine of union with Christ is true?

The doctrine of union with Christ is an essential tenet in Reformed theology, articulated in passages such as John 15, where Jesus invites His followers to remain in Him. This union not only signifies a close relationship but also asserts that believers share in His life, suffering, and glory. The love and desire Jesus expresses for His disciples highlight the reality of this union—He longs to be with them and share His life with them. Moreover, this relationship goes beyond mere ideological alignment; it signifies a transformative experience where believers partake in the divine grace that flows from Christ, thus solidifying the truth of this doctrine in the believer's life.