In other words, in our union with Christ, we receive not only the benefit of justification, but we also have the benefit of sanctification. So many people think that their sanctification, their spiritual transformation and conformation to the holy image of Christ, is simply a matter of trying harder, of pulling as hard as they can on their moral bootstraps—resolving to be holier. However, one thing that should be clear is that Jesus clearly tells us that the only way we will produce fruit is if we abide in Him: “I am the vine; you are the branches. Whoever abides in me and I in him, he it is that bears much fruit, for apart from me you can do nothing” (John 15:5).
One of the most breath-taking passages of Scripture appears in the opening of Paul’s epistle to the Ephesians, where the Apostle literally starts at the very beginning when he writes, “In love he [that is, God] predestined us for adoption as sons through Jesus Christ” (1:4–5). As Paul unfolds all of the blessings that believers receive, he anchors salvation in Christ with the repetition of a phrase: “In him . . .” Paul writes, “In him we have redemption through his blood, the forgiveness of our trespasses . . . to unite all things in him. . .. In him we have obtained an inheritance. . .. In him you also . . . were sealed with the promised Holy Spirit” (vv. 7–13, emphasis mine). Paul repeats the refrain “in him,” which points us to the doctrine of union with Christ. But what exactly is union with Christ?
In his Systematic Theology, Louis Berkhof defines union with Christ as “that intimate, vital, and spiritual union between Christ and His people, in virtue of which He is the source of their life and strength, of their blessedness and salvation.” There are a number of passages throughout the Scriptures that reveal believers are joined to Christ: We are the branches and Jesus is the vine (John 15:5); Jesus is the head and we are His body (1 Cor. 6:15–19); Christ is the foundation and we are living stones joined to the foundation (1 Peter 2:4–5); and marriage between a husband and wife ultimately points to the union between Christ and believers (Eph. 5:25–31). Beyond these biblical images, the specific phrase “in Christ” occurs some twenty-five times in Paul’s epistles. We can say that union with Christ entails all of the benefits of our redemption. Westminster Larger Catechism question 69, for example, asks, “What is the communion in grace which the members of the invisible church have with Christ?” It then responds, “The communion in grace which the members of the invisible church have with Christ, is their partaking of the virtue of his mediation, in their justification, adoption, sanctification, and whatever else, in this life, manifests their union with him.”
The Larger Catechism’s answer is easily verified from Scripture. For example, as we saw above, we are chosen “in him” before the foundation of the world (Eph. 1:4). Paul writes to the church at Rome that there is “no condemnation for those who are in Christ Jesus” (Rom. 8:1), which is another way of saying that those who are united to Christ are justified. Anyone who is “in Christ Jesus” is a son of God through faith (Gal. 3:26). Moreover, if Christians abide in Christ, they will bear much fruit; they will produce good works (John 15:5). Only Christ gives us our salvation, whether considered as a whole or as the different individual benefits, such as justification and sanctification.
What is the significance of the fact that believers are united to Christ? Reformed theologians have historically argued that there are a number of different aspects to our union with Christ. For example, we are united to Christ in terms of our election “in him.” We were not indwelt by the Holy Spirit at this point and united to Christ by faith because we did not even exist except in the mind of God. Nevertheless, we are united to Christ in terms of the Father’s decision to elect individual fallen sinners and redeem them through His Son. Hence, in this sense, we are united to Christ in the decree of election.
There is a second aspect of union with Christ, which some have called our representative or federal union. In Christ’s earthly ministry, everything that He did, He did in behalf of His bride, the church. When He was baptized in the Jordan River, which was a baptism of repentance, He was not confessing personal sin, as He was the spotless Lamb—He was sinless (Mark 1:4; 1 Peter 1:19). Rather, as His people’s representative, He was acting in their behalf. Consequently, not only in His baptism but in His fulfillment of every jot and tittle of the law, in His perfect suffering, resurrection, and ascension—everything that Christ did was in behalf of His bride. Christ’s perfect law-keeping and suffering become ours through faith—they are imputed, or accredited, to us. Christ’s resurrection is representative, in that as the head is raised, so the body, the church, will be raised in precisely the same manner. Now, as Christ sits in royal session at the right hand of His heavenly Father, we are seated with Christ and rule with Him in the heavenly places (Eph. 1:20–21).
A third aspect of our union with Christ is what some call the mystical or personal union. This is the personal indwelling of the believer by faith through the person and work of the Holy Spirit. A number of passages speak to this dimension of our union with Christ, including Ephesians 2, where the Apostle Paul explains that we are members of the household of God, built on the foundation of the Apostles and prophets, with Christ as the cornerstone. About this grand and final temple, Paul writes that we grow “into a holy temple in the Lord,” and “in him [we] also are being built together into a dwelling place for God by the Spirit” (v. 22).