The Westminster Larger Catechism, 65 through 69, describes, in part, union with Jesus Christ. And John Calvin in Book Three of Institutes of the Christian Religion describes union with Christ, among other things, as a spiritual union. And that union between Christ and the Christian is effected by the secret energy or power—the work of the Holy Spirit. So, in biblical texts like Ephesians 3:16–17—the Spirit dwells in you, Christ dwells in your heart the spirit joins Christ to the Christian and the Christian to Christ—that’s true individually and then corporately as the body of Christ. But not only is union with Christ in life experience affected by the Spirit, it’s affected by the Spirit as the Spirit grants or produces faith in the Christian. So that union with Christ has the Spirit as its bond from the divine side; union with Christ has Spirit-produced faith as the bond of union from the human side.
That faith is a gift from God (Eph. 2:8). It is not by works. It does not have its origin in the creature. It has its origin as a gift from God. And Philippians 1:6 says that the one who began this good work of Spirit-generated faith will carry it on to completion until the day of Jesus Christ. Now that faith, union with Christ by the Spirit, can further be qualified most basically as a union with Christ, the person, so it’s a spiritual union. It’s a union by faith, and it’s a union with the person of Jesus Christ, crucified and raised. Paul can say that in Christ the church has every spiritual blessing in heavenly places. There are more things we could say about union with Christ but present personal union with Christ is a union that is produced by the Spirit, through faith, with the person of the crucified and ascended Christ. And it consists in a bond of vital, reciprocal, never-ending, always ascending fellowship with Jesus Christ in grace in this age and in glory in the age to come.
Adapted from a transcript of the video.