What is meant by 'eternal union' in Christian theology?

In Christian theology, 'eternal union' signifies the belief that God's elect were united with Christ before the foundation of the world. Ephesians 1:4 tells us that believers were chosen in Christ before the world was created, highlighting that our relationship with Him transcends time. This union is attributed to God's sovereign grace, meaning it was not founded upon human effort or will, but purely on God's purpose and plan. The implications of this doctrine are profound; it provides assurance of salvation and underscores the steadfastness of God's promises to His people.