What is that union which the elect have with Christ?
The union which the elect have with Christ is the work of God’s grace, (Eph. 1:22, Eph. 2:6–7) whereby they are spiritually and mystically, yet really and inseparably, joined to Christ as their head and husband; (1 Cor. 6:17, John 10:28, Eph. 5:23,30) which is done in their effectual calling. (1 Pet. 5:10, 1 Cor. 1:9)
The Westminster Larger Catechism: With Scripture Proofs. (Oak Harbor, WA: Logos Research Systems, Inc., 1996).