Union with Christ is one of the central theological principles of the Christian faith. Its pervasive presence in the New Testament typically is indicated by the word in, a simple preposition with profound implications.
Believers often are said to be in Christ: "If anyone is in Christ, he is a new creation" (2 Cor. 5:17). Sometimes this phrase passes by so rapidly that we may hardly notice, as in Paul's opening address to "the saints in Christ Jesus who are at Philippi" (Phil. 1:1). But even such passing expressions are grounded in the deep spiritual truth of our faith-union with Jesus Christ. The reason we are called "saints in Christ" is because our true and ultimate identity is found in Him: "you are all one in Christ Jesus" (Gal. 3:28).
On other occasions, the Bible teaches the reciprocal principle that Jesus Christ is in the believer: "It is no longer I who live, but Christ who lives in me" (Gal. 2:20). Similarly, Paul wrote of the gospel mystery that has been "hidden for ages and generations but now revealed to his saints" (Col. 1:26). What is this glorious mystery? "Christ in you, the hope of glory" (Col. 1:27).
Christ is in us and we are in Christ. The two sides of this mutual relationship sometimes appear together in Scripture. For example, in teaching His disciples about the vine and the branches—a metaphor for union with Christ—Jesus said, "Abide in me, and I in you" (John 15:4). Similarly, the apostle John described union with Christ as a double habitation by the Holy Spirit: "We know that we abide in him and he in us, because he has given us of his Spirit" (1 John 4:13).
By virtue of this mutual relationship of spiritual indwelling—our union with Christ—we receive all the saving blessings of God. In being united to Christ, we receive not only Christ Himself, but also His benefits. What is His becomes ours, for God "has blessed us in Christ with every spiritual blessing" (Eph. 1:3). Thus we see, said Calvin, that "our whole salvation and all its parts are comprehended in Christ." Indeed, union with Christ is the heart of the gospel, for when the apostle Paul "defines the Gospel, and the use of it, he says that we are called to be partakers of our Lord Jesus Christ, and to be made one with Him, and to dwell in Him, and He in us; and that we be joined together in an inseparable bond."
When Calvin considered how "we receive those benefits which the Father bestowed on his only-begotten Son," his answer was that we receive them by our union with Christ. Christ must "present Himself to us and invite us into such a relationship that truly we are united to Him, that He dwells in us in such a way that everything that belongs to Him is ours." Thus Calvin made union with Christ one of the controlling principles of his soteriology, or doctrine of salvation.
Apart from union with Christ, it is impossible to receive any of the saving blessings of God. Not even the cross and the empty tomb can save us unless we are joined to Jesus Christ. Calvin was emphatic:
We must understand that as long as Christ remains outside of us, and we are separated from him, all that he has suffered and done for the salvation of the human race remains useless and of no value for us. Therefore, to share with us what he has received from the Father, he had to become ours and to dwell within us…. We also, in turn, are said to be "engrafted into him" [Rom. 11:17], and to "put on Christ" [Gal. 3:27]; for, as I have said, all that he possesses is nothing to us until we grow into one body with him.
Simply put, if we are not in Christ, we have no part in His death on the cross to atone for sins and no share in His resurrection from the dead. We are not justified, adopted, sanctified, or glorified without being united to Christ. "I do not see," wrote Calvin, "how anyone can trust that he has redemption and righteousness in the cross of Christ, and life in his death, unless he relies chiefly upon a true participation in Christ himself. For those benefits would not come to us unless Christ first made himself ours." Union with Christ, therefore, is nothing less than a matter of spiritual life and death.
This excerpt is taken from John Calvin: A Heart for Devotion, Doctrine, and Doxology edited by Burk Parsons.