Philip Ryken writes:
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…