Westminster Q66: What is that union which the elect have with Christ?

A66: 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]

(Eph 1:22) And He put all things in subjection 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 ages to come He might show the surpassing riches of His grace in kindness toward us in Christ Jesus. For by grace you have been saved through faith; and that not of yourselves, it is the gift of God;
(1Co 6:17) But the one who joins himself to the Lord is one spirit with Him.

(Joh 10:28) and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand.

(Eph 5:23) For the husband is the head of the wife, as Christ also is the head of the church, He Himself being the Savior of the body.

(Eph 5:30) because we are members of His body.
(1Pe 5:10) After you have suffered for a little while, the God of all grace, who called you to His eternal glory in Christ, will Himself perfect, confirm, strengthen and establish you.

(1Co 1:9) God is faithful, through whom you were called into fellowship with His Son, Jesus Christ our Lord.