Marcus Peter Johnson in One with Christ: An Evangelical Theology of Salvation [Illinois: Crossway, 2013] explains:
To say that our union with Christ is Trinitarian means that by virtue of being incorporated into the life of Jesus Christ, we participate in the life, love, and fellowship of the Trinity. Because the Son is one with the Father, our being joined to the Son means we are joined to the Father. And because the Spirit exists as the bond of communion between the Father and Son, he brings us into that communion by uniting us to Christ. This staggering biblical revelation forms the personal foundation for all the benefits that constitute our salvation.
Augustine opens his Confessions with one of the best -known passages in Christian literature: "For you [God] have made us for yourself , and our heart is restless until it rests in you." Calvin likewise affirms that the perfection of human happiness "is to be united with God." Both were expressing a basic scriptural truth— the greatest need and desire (whether conscious or not) of human beings, fallen and estranged from God, is to be restored to the One who created us and loves us, and apart from whom we perish. This is precisely what our union with the God-man Jesus Christ accomplishes. The apostle John records various ways in which Jesus spoke of this reality: "I am the way, and the truth, and the life. No one comes to the Father except through me" (John 14: 6); "Believe me that I am in the Father and the Father is in me" (v. 11). Jesus speaks of including believers in the intimacy he has with his Father: "The glory that you have given me I have given to them, that they may be one even as we are one, I in them and you in me" (17: 22– 23); "I made known to them your name, and I will continue to make it known, that the love with which you have loved me may be in them, and I in them" (v. 26).
As staggering and incomprehensible as it may seem, our union with Christ is a union with the triune God, for Christ incorporates us into his relationship with his Father. And because Christ is identical in nature (homoousion) with the Father, to be united to the person of Christ (through his incarnate humanity) is to be united to the whole Christ, the One who is fully human and fully God in one person. As it turns out, the doctrine of the hypostatic union is more than mere christological ontology— it tells us that the One who took on our flesh unites that flesh indivisibly to his deity so that we experience fellowship with our Maker again. This is good news indeed!
For the Word of God is a divine nature even when in the flesh, and although he is by nature God, we are his kindred because of his taking the same flesh as ours. Therefore the manner of the fellowship is similar. For just as he is closely related to the Father and through their identity of nature the Father is closely related to him, so also are we [closely related] to him and he to us, in so far as he was made man. And through him as through a mediator we are joined to the Father.
I will say a word below about why being joined to God the Father through Christ does not mean that Christians become God or gods (deification). We are affirming here that the union of our humanity with the humanity of Christ necessarily involves our sharing in the whole person of Christ as divine and human . To be united to Christ is therefore to share in his oneness with the Father. We are united to God the Father through God the incarnate Son.
Furthermore, we are united to Christ by the Holy Spirit. Theologians have often expressed this truth by referring to the union as "spiritual." We must take care when we express ourselves this way, however. Speaking of a "spiritual" union— as opposed to speaking, as we should, of a union with Christ that occurs by way of the third person of the Trinity— might conjure up some conceptions that are problematic and unbiblical. One potential problem is that referring to our union with Christ as "spiritual" risks obscuring the specific, essential reality of the union (Christ himself) in ambiguous religious sentiment. Another is that the term spiritual may imply that the union is merely between Christ's Spirit and our spirits, as if the union consisted in our shared convictions or dispositions. Finally, and perhaps most dangerous of all, references to a "spiritual" union may be taken to imply that the union occurs between believers and the Holy Spirit in abstraction from Jesus Christ. According to this view, the indwelling Holy Spirit functions as a replacement for the absent Christ, and it has been my experience that many Christians think of the Spirit in this way. These views need to be corrected.
To say that our union with Christ occurs by the power of the Spirit means that the Holy Spirit is himself the bond who unites us to the living Christ. Christ sent the Spirit not so that we might have a roughly suitable replacement in his absence, but that we might enjoy the actual presence of Christ (through the Spirit). The Spirit is the personal manner or mode of Christ's dwelling in us. Thus, in the Scriptures we see that the presence of the Spirit is closely associated with the presence of Christ, and the Spirit is even called the "Spirit of Christ" (Rom. 8: 9– 11; cf. Gal. 4: 6; Phil. 1: 19). Christ's sending of the Spirit means that, through the indwelling of the Spirit, Christ is in us and we are in him (John 14: 16– 20). Notice his words in this passage: "I will not leave you as orphans; I will come to you." Jesus did not send an alternative but his very presence through the Spirit. The Spirit did not come to mediate his own presence, to glorify his own name, to teach us about himself, or to form the body of the Spirit. The Spirit was not incarnated, crucified, or resurrected for our salvation. The Spirit came to make Christ known , to glorify Christ's name, to teach us about Christ, and to form us together as the body of Christ (John 15: 26; 16: 14– 15; 1 Cor. 2: 14– 16; 12: 13).
The heart of the Spirit's ministry is to join us to the incarnate, crucified, resurrected, ascended, and living Lord Jesus Christ. J. I. Packer writes that "the distinctive, constant, basic ministry of the Holy Spirit in the New Covenant is . . . to mediate Christ's presence to believers." Therefore, describing union with Christ as a "spiritual" union can mean only that it is a union with Christ that takes place through the power of the Holy Spirit— it is a Spiritual union . After all , the essential point of the Spirit's coming is not to unite us to the Spirit, but to bring us the Savior:
Nothing, therefore, is bestowed on us by the Spirit apart from Christ, but he takes it from Christ, that he may communicate it to us . . . for he does not enlighten us to draw us away in the smallest degree from Christ. . . . In a word, the Spirit enriches us with no other than the riches of Christ, that he may display his glory in all things.
Thus, our union with Christ is fully Trinitarian, a participation in the relations of the triune God. In our union with the Word of God, Jesus Christ, we are beneficiaries of the perfect union of God and man in himself, and we now share in his oneness with the Father. And the same Holy Spirit— in the words of the Nicene Creed, "the Lord, the Giver of life"— who conceived the incarnate Christ also conceives us anew by incorporating us into the One who is life in himself.
15. In the words of Jonathan Edwards: “There was, [as] it were, an eternal society or family in the Godhead, in the Trinity of persons. It seems to be God’s design to admit the church into the divine family as his son’s wife” (cited in Brad Harper and Paul Louis Metzger, Exploring Ecclesiology: An Evangelical and Ecumenical Introduction [Grand Rapids: Brazos Press, 2009], 37).
16. Augustine, Confessions, trans. Henry Chadwick (Oxford: Oxford University Press, 1998), 1.1.
17. Calvin, Institutes, 1.15.6.
18. Cyril of Alexandria, Commentary on John (cited in Donald Fairbairn, Life in the Trinity: An Introduction to Theology with the Help of the Church Fathers [Downers Grove, IL: IVP, 2009], 138, emphasis added).
19. It is important to stress that our union with God is mediated by the incarnate humanity of Christ, not only because this is the theo-logic of the incarnation, but also so that we avoid speaking in an unqualified sense of a union with God in his very essence or substance.
20. Often, the term spiritual is used as a synonym for religious to describe someone’s religious disposition (whether Christian or not). When the apostle Paul uses the term, it is nearly always a reference to the third person of the triune God, necessarily excluding non-Christians.
21. I suspect this is because of a truncated reading of John 14– 17 and Acts 2, where the sending of the Spirit is interpreted as something other than Christ’s presence through the Spirit. This misreading is often reinforced by notions of Spirit baptism that fail to stress that the Spirit baptizes believers into Jesus Christ. Familiar choruses we sing may also reinforce such notions: “Thank you, oh my Father, for giving us your Son, and leaving your Spirit ‘til the work on earth is done” (from the song “There Is a Redeemer,” written by Melody Green; © Universal Music Publishing Group).
22. J. I. Packer, Keep in Step with the Spirit (Grand Rapids: Fleming H. Revell, 1984), 49.
23. Calvin, Commentaries, John 16: 14.