The union isn’t physical, but theological. Union with Christ implies three things: solidarity (Christ as the second Adam is our representative), transformation (Christ by the Holy Spirit changes us from the inside out), and communion (Christ abides with us as our God). Union with Christ is like wedlock, where we are joined to Christ in a covenant of love. It is like a body where we as members are joined to our living Head. Or you might say union with Christ is like a building, where we are the house and Christ dwells within us.