“I am the vine you are the branches.” John 15:5
It is a most close and INTIMATE union. Believers, regenerate people, who believe in him, and rely on him, have put on Christ, Gal. 3:27. If that be not enough, he is in them, John 17:23, formed in them as the child in the womb, Gal. 4:19. He is the foundation, 1 Cor. 3:11; they are the living stones built upon him, 1 Pet. 2:5. He is the head, and they the body, Nay, he lives in them, as their very souls live in their bodies, Gal. 2:20. And what is more than all this, they are one in the Father and the Son, as the Father is in Christ, and Christ in the Father, John 17:21, “That they all may be one; as thou Father art in me, and I in thee, that they also may be one in us.”
Though it is not a mere LEGAL union—yet it is a union supported by law. Christ, as the surety, and Christians as the principal debtors, are one in the eye of the law. When the elect had run themselves, with the rest of mankind, in debt to the justice of God, Christ became surety for them, and paid the debt. When they believe on him, they are united to him in a spiritual marriage union; which takes effect so far, that what he did and suffered for them is reckoned in law, as if they had done and suffered it themselves. Hence, they are said to be crucified with Christ, Gal. 2:20; buried with him, Col. 2:12; yes, raised up together, namely, with Christ, “and made to sit together in heavenly places in Christ Jesus,” Eph. 2:6. In which places, saints on earth, of whom the apostle there speaks, cannot be said to be sitting—but in the way of law reckoning.
It is a MYSTERIOUS union. The gospel is a doctrine of mysteries. It discovers to us the substantial union of the three persons in one Godhead, 1 John 5:7, “These three are one;” the hypostatic union, of the divine and human natures, in the person of the Lord Jesus Christ, 1 Tim. 3:16, “God was manifest in the flesh;” and the mystical union, between Christ and believers; “This is a great mystery” also, Eph. 5:32. O, what mysteries are here! The head in heaven, the members on earth—yet really united! “Christ in the believer, living in him, walking in him:” and “the believer dwelling in God, putting on the Lord Jesus, eating his flesh, and drinking his blood!” This makes the saints a mystery to the world; yes, a mystery to themselves.