Union with Christ reminds us we cannot lay hold of one benefit of Christ without receiving the others. We cannot come to Christ for justification and somehow sneak away without sanctification. To have one benefit is to have Christ. To have Christ is to have all benefits. The benefits of justification and sanctification are, of course, distinct from each other, but they are not distinct from the one communion in grace the believer has by virtue of union with Christ.
That holiness of life which the Christian has from God and before God and for God is not sourced nor drawn from even the best doctrinal formulations – as essential as they are to our faith. Nor is holiness of life sourced or drawn from moral transformation – as essential as it is to living out our faith. Holiness of life is drawn from Christ, who alone is “life-giving Spirit” (1 Cor. 15:45).
You see, we may have doctrinal formulations and moral pursuits but still be outside of Christ. How? Because the life Christ bestows does not spring up from the work of our own hands nor does it spring out of our own minds. True life is from above not below. It comes from heaven in the person of Jesus Christ (John 6:33).
The Westminster Larger Catechism asks, “What is that union which the elect have with Christ?” (Q. 66).
Answer: “The union which the elect have with Christ is the work of God’s grace, whereby they are spiritually and mystically, yet really and inseparably, joined to Christ as their head and husband; which is done in their effectual calling.”
When the soul dead in sin is effectually called, the Spirit bestows saving faith and puts us into Christ and puts Christ into us (John 14:20). “In him” is the apostle Paul’s ubiquitous shorthand for the doctrine of union. The Spirit takes what belongs to Christ and gives it to us. We are not just given the Spirit, as it were, but Christ gives us himself through the Spirit. Because we now have him, the many wonderful benefits of his mediation become ours.
Again, the Westminster Larger Catechism asks: “What is the communion in grace which the members of the invisible church have with Christ?” (Q. 69). Answer: “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.”