We see a comprehensive sweep of our union with Christ from eternity to eternity:
• Eternal origin: Predestinated in Christ (Eph 1:4)
• Eternal end: Glorified in Christ (Rom 8:17, 1 Cor 15:22)
Gaffin then makes three categorical distinctions of this union:
• Predestinarian union with Christ (Eph 1:4).
• Redemptive-historical union with Christ (Rom 6:1–14). The union that is involved as we are seen as one with Christ when he actually accomplished our salvation. We are crucified, buried, raised up with Christ.
• Applicatory (or existential) union with Christ. Paul percieved himself as one chosen in Christ from eternity (1) and as one who was contemplated in Christ during his death/resurrection (2). But Paul also knew that he was at one time NOT united to Christ, but was rather a child of wrath (Rom 16:7). So what effects the transition from wrath to grace? That point came when Paul was united to Christ by faith.
These are not three distinct unions but three facets to the single union.
Under “applicatory union” Gaffin makes these further points:
• Mystical union. It is mystical union because it involves a great mystery, a mystery that has its closest analogy in the relationship between a husband and a wife (Eph 5). Marital union and intimacy does not blur the distinctions between the husband and wife. So our union with Christ does not blur the clear personal distinction between Christ and the Christian. Christ remains our representative. This point protects us from mysticism.
• Spiritual union. It is spiritual because of the activity and indwelling of the Holy Spirit. This union is not ontological (like the Trinity), not hypostatic (like the two natures of Christ), not psychosomatic (body-soul relationship), not one flesh (like marriage), nor is it merely intellectual and moral (as if Christ and the believe now merely share a common purpose). Spiritual union is rooted in the relationship between Christ and the Holy Spirit.
• Reciprocal union. Believers are in Christ and Christ is also in them. The hope of the church is that Christ “is in you” (Col 1:27).
• Vital union. Christ’s indwelling is the very life of the believer (Gal 2:20, Col 3:4).
• Permanent union. Rooted in election, our union will reach its final consummation in glorification. At the end of Rom 8 Paul says that nothing, not even death, can separate us from the love of Christ. Why? Because not even death can separate us from Christ. Westminster Confession: “The souls of believers are at their death made perfect in holiness, and do immediately pass into glory; and their bodies, being still united to Christ, do rest in their graves till the resurrection.”
• Corporate union. Union is obviously very personal. But it’s also corporate (1 Cor 1:9, 12:13). The call that comes to each believer is also a call into fellowship with His Body. There is no union that is not also fellowship with other believers. Never polarize the personal and corporate concerns.
• Union and justification. We do not have our justification apart from, or prior to, our being united to Christ. Justification is a manifestation of our union with Christ.
John Calvin, Institutes, 3.11.10: “that joining together of Head and members, that indwelling of Christ in our hearts—in short, that mystical union—are accorded by us the highest degree of importance, so that Christ, having been made ours, makes us sharers with him in the gifts with which he has been endowed. We do not, therefore, contemplate him outside ourselves from afar in order that his righteousness may be imputed to us but because we put on Christ and are engrafted into his body—in short, because he deigns to make us one with him.”
Conclusion. A focus on our union with Christ will keep our focus on Christ himself, on his person, rather than being preoccupied merely by the benefits we receive from Him. It keeps Christ central. It will not allow him to fade into the background as a mere facilitator of these benefits.