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,[1] whereby they are spiritually and mystically, yet really and inseparably, joined to Christ as their head and husband;[2] which is done in their effectual calling.[3] 

1. Eph. 1:22

   And he put all things under his feet and gave him as head over all things to the church… .

   Eph. 2:6-8

   … and raised us up with him and seated us with him in the heavenly places in Christ Jesus, so that in the coming ages he might show the immeasurable riches of his grace in kindness toward us in Christ Jesus. For by grace you have been saved through faith. And this is not your own doing; it is the gift of God … .

2. I Cor. 6:17

   But he who is joined to the Lord becomes one spirit with him.

   John 10:28

   I give them eternal life, and they will never perish, and no one will snatch them out of my hand.

   Eph. 5:23, 30

   For the husband is the head of the wife even as Christ is the head of the church, his body, and is himself its Savior.
   
   … because we are members of his body.

3. I Peter 5:10

   And after you have suffered a little while, the God of all grace, who has called you to his eternal glory in Christ, will himself restore, confirm, strengthen, and establish you.

   I Cor. 1:9

   God is faithful, by whom you were called into the fellowship of his Son, Jesus Christ our Lord.

Question 66, Westminster Larger Catechism