The doctrine of union with Christ is all throughout the Pauline corpus but 2 Cor. 5:16-21 seems to be the most explicit of Paul’s earlier letters. Christ is the operative word in our passage. Everything happens in and through Him. So, it seems good to ask: “How can being ‘in Christ’ have the effect that it has?” However, as we will see the answer to that question is: “How could being ‘in Christ’ not have cosmic significance?!”
Christ’s work and resurrection propels on this world new creation (cf. Rom. 8:29; 1 Cor. 15:20; Col. 1:18), it is the inevitable avalanche that will eventually encompass the whole earth (Ps. 72:19; Is. 11:9; Hab. 2:14) and those in Christ will be swallowed up in the effulgence of its glory, there to bask in eternal joy. Christ’s resurrection is the dawn, the first light, but soon the full splendor of the sun.
We will first see how union with Christ is foreshadowed in the OT which will help us substantially to understand the full significance, indeed the cosmic significance, of being “in Christ.” Then we will see that 2 Corinthians 5:16-21 teaches us that union with Christ is the means by which reconciliation—cosmic, corporate, and individual—happens.
“In Christ” was “unquestionably one of Paul’s favorite phrases, appearing 164 times in the chief Pauline letters and another half dozen in the form en Christō Iēsou (“in Christ Jesus”) in the Pastorals.” John Murray says, “Nothing is more central or basic than union and communion with Christ.” The idea of being in Christ or having union with Christ is communicated in various ways and means various things. This makes it difficult to define and explain but helps us to see the doctrines huge significance (especially in Paul).
Understanding the precedence for Paul’s doctrine is crucial because without doing so we will not be able to truly understand the full significance of Paul’s words. It is crucial because the OT provides the foundation for every NT teaching, including union with Christ. “If the New Testament relies on the Old as the basis for its theological principles, then it makes sense that union with Christ does not emerge from a void but rather fills out concepts introduced in the Old Testament.” So, we want to briefly see some of those concepts and principles.
I do not want to be guilty of seeing union with Christ in everything but I do believe we see union with Christ foreshadowed in many places when we are looking. Peterson lists out three different ways that union with Christ is foreshadowed in the OT: identification, God’s covenant presence with His people; incorporation, membership in God’s covenantal people; and lastly participation, sharing in the covenantal story. Campbell says, “The conceptual antecedents for Paul’s thought are to be found in Jewish theology, the Old Testament, and the words of Jesus, beginning with his encounter with Christ on the Damascus Road.”
We can see the concept of representative head (or corporate solidarity) in the OT and certainly from the NT perspective we know we are either in Adam or in Christ as our representative head. However, I believe we see solidarity with Christ foreshadowed in many ways. We see solidarity foreshadowed not just with Adam,  Abraham, and David but also in the sacrifices that were offered.
The OT people of God were in a sense “in” the animal that was sacrificed because it was their sin and it was they that deserved death. However, their sin was imputed to the animal and so its death was in a sense their death. In a similar way, the priest, though not inherently undefiled, “sprinkled” people to atone for sin (Ex. 29:21; Is. 52:15; 1 Pet. 1:2). So, we see, the priest, as a sort of head, mediator, and representative brought reconciliation, if only temporary.
The thing is, Jesus the Christ, is not only the Second (and better) Adam, He is also the Prophet, the Priest, the King, and the Lamb of God that takes away the sin of the world (Jn. 1:29). The idea of union was foreshadowed but never had all these positions (that had a connection to bringing reconciliation) been combined. So, the Pauline doctrine of union with Christ was not quite unprecedented, it was just not seen or understood yet. However, all the furniture was in the room, so to speak, but it was not seen for what it was because the light of revelation had not revealed it.
If union with Christ is foreshadowed in the OT why do we not see it in other places besides in the Pauline corpus? First, Paul, as a zealous Pharisee, had much more training in the OT Scriptures than did the disciples so it makes sense that the doctrine of union with Christ would come out the most in his writings. Paul wrote with a different purpose than did the Synoptic Gospel writers, for instance. Paul’s writings are not biographical but more theological so it makes sense that he would expound on union with Christ while other writers would not.
It also seems significant that the Gospel of John, the “theological gospel,” has the most to say about union with Christ. We also see union with Christ come out in his letters as well. Perhaps this is because of John’s background or longer life. We do not know. It seems clear to me though that the idea of union with Christ is not unprecedented in Scripture, whether in the Old Testament or in other places in the New Testament. However, that is not to say that Paul did not shine a bright light on what was before only a very dark shadow.
Second, there are a few places in the NT other than in Paul’s writings where we see union with Christ. The obvious example is in John’s writings (Jn. 6:56; 15:4, 5, 7; 1 Jn. 4:13). However, we also see the concept of union with Christ briefly in Peter (1 Pet. 1:11; 5:14) and Hebrews (Heb. 3:12-14).
Understanding what foreshadows the doctrine of union with Christ helps us make sense of how union with Christ brings reconciliation. If we remember who Christ is and how he succeeded where everyone else finally failed then it helps us to understand how He can bring cosmic as well as personal reconciliation. David was in a sense commissioned with the task of bringing shalom but he brought shame. Solomon too with his wisdom did bring a time of shalom but in the end split the kingdom asunder. Hoping in an earthly king would not bring the forever kingdom that was promised and hoped for (2 Sam. 7:13, 16; Ps. 89:4, 29, 36-37).
As we take in the full panoramic view of how union with Christ was foreshadowed we see the beauty of union with Christ with new and broader appreciation. We will wonder anew at how Christ is the fulfillment of Scripture and the many promises therein. We will also have our present text come alive with fresh insight as we see better the connections that it has to the whole story of Scripture. It will help us change our myopic vision and put on glasses that help us to see the full wonderful scope of the new creation work that is accomplished in Christ.
Understanding the precedence for union with Christ helps us to understand how God could be in Christ reconciling the “world,” that is, is the physical and tangible creation, to Himself (v. 19). Again, in Adam, there is curse, confusion, and chaos, but in Christ, there is new creation. In Adam there is death but in Christ there is life (cf. Rom. 5).
 See e.g. Michael Horton, The Christian Faith: A Systematic for Pilgrims On the Way (Grand Rapids: Zondervan, 2011), 590.
 See 2 Cor. 5:17 “in Christ” (ἐν Χριστῷ), v. 18 “through Christ” (διὰ Χριστοῦ), v. 19 “in Christ” (ἐν Χριστῷ, v. 21 “in Him” (ἐν αὐτῷ).
 See e.g. 1 Cor. 15:24-28; Col. 1:15-20.
 “The resurrection of Christ in fact means the breakthrough of the new aeon in the real, redemptive-historical sense of the word, and therefore cannot be understood only in forensic, ethical, or existential categories. This all-embracing significance of the resurrection of Christ is in Paul likewise not only the fruit of his profound theological reflection, but about all of divine revelation. For, as he himself expresses it, when it pleased God to reveal his Son to him (Gal. 1:15), that was first and foremost the evidence for him that Jesus of Nazareth, who was crucified and had died and whom he himself had persecuted, was the Son of God and the Messiah of Israel. And it was this certainty, entirely foreign and even offensive to Jewish thinking, which determined his insight into the redemptive-historical significance of Christ’s death and resurrection in a decisive manner. Because Jesus was the Christ, his resurrection is not, as previous raisings of the dead, an isolated occurrence, but in it the time of salvation promised in him, the new creation, dawns in an overwhelming manner, as a decisive transition from the old to the new world (2 Cor. 5:17; cf. v. 15)” (Herman N. Ridderbos, Paul: An Outline of His Theology trans. John Richard De Witt [Grand Rapids: Wm. B. Eerdmans Publishing Co., 1975], 55-56).
 “The expression ‘in Christ’ is one of Paul’s most characteristic formulations and its precise meaning has been vigorously debated” (George Eldon Ladd, A Theology of the New Testament [Grand Rapids, William B. Eerdmans Publishing Co., 1993], 523). Union with Christ is often referred to as “mystical union” because union with Christ encompasses so much and is hard to define. A. A. Hodge says it is designated “mystical” because it “transcends all the analogies of earthly relationship, in the intimacy of its communion, in the transforming power of its influence, and in the excellence of its consequence” (Horton, The Christian Faith, 590 see also “Defining ‘Union with Christ,’ 406-20 in Constantine R. Campbell’s Paul and Union with Christ: An Exegetical and Theological Study [Grand Rapids: Zondervan, 2012).
 Philip Edgcumbe Hughes, Paul’s Second Epistle to the Corinthians (Grand Rapids: Wm. Bible Eerdmans Publishing Co., 1962), 202. See also John Calvin, Institutes, 2.16.19.
 B. Witherington III, “Christ” in Dictionary of Paul and His Letters, ed. Gerald F. Hawthorne, Ralph P. Martin, and Daniel G. Reid (Downers Grove: InterVarsity Press, 1993), 98.
 John Murray, Redemption Accomplished and Applied (Grand Rapids: Wm. B. Eerdmans Publishing Co., 1955), 161. “As far back as we can go in tracing salvation to its fountain we find ‘union with Christ’; it is not something tacked on; it is there from the outset” (Ibid., 162). “Communion with God is the crown and apex of true religion” (Ibid., 170). “Union with Christ is the central truth of the whole doctrine of salvation” (Ibid.).
 Robert A. Peterson, Salvation Applied by the Spirit: Union with Christ (Wheaton: Crossway, 2015), 19.
 Peterson, Salvation Applied by the Spirit, 20. Truly, “to be biblical we must first do justice to the Old Testament context, and in the case of typology we must discern whether something is a legitimate type/pattern. Then we must think through that pattern’s intertextual development across the biblical covenants, and then finally ask how it is brought to fulfillment in Christ and the arrival of the new covenant age” (Peter J. Gentry and Stephen J. Wellum, Kingdom through Covenant: A Biblical-Theological Understanding of the Covenants [Wheaton: Crossway, 2012], 607).
 See Peterson, Salvation Applied by the Spirit, 20-32 see also Campbell, Paul and Union with Christ, 406-20. In my reading, Cambell has given the fullest explanation and definition of what it means to be “in Christ.”
 Campbell, Paul and Union with Christ, 420.
 I was helped by Richard M. Davidson’s working paper “Corporate Solidarity in the Old Testament” (http://www.gospelstudygroup.org/wp-content/uploads/2010/08/corporate-solidarity-in-OT.pdf). Richard M. Davidson is J. N. Andrews Professor of Old Testament Interpretation at Andrews University.
 Though it should be understood that not everyone accepts this idea of federal headship. See e.g. J.W. Rogerson, “The Hebrew Conception of Corporate Personality: A Re-examination” in JTS 21 (1970) and Andrew Perriman, “The Corporate Christ: Re-assessing the Jewish Background” in Tyndale Bulletin 50.2 (1999) 239-63.
 “For Paul, all humankind is automatically ‘in Adam’, it would appear that the state of being ‘in Christ’ is brought about through the faith and baptism of individuals (Gal 3.26-28)” (Margaret E. Thrall, The Second Epistle to the Corinthians [Edinburgh, Scotland: T&T Clark, 1994], 410).
 Of course, one of the differences was that none of those animals rose victoriously out of the grave. None of the animals conquered death. None of the animals inaugurated the new creation.
 “From creation to the flood to the exodus all the way to the new creation, there is a close connection between the covenant and union with the covenant mediator” (Horton, The Christian Faith, 589).
 It seems also that marriage language is perhaps a foreshadowing. Husband and wife certainly have union and Israel is said in various places to be God’s bride, even if unfaithful (cf. Hosea). Of course, this is an example of a type of anthropomorphic language but we perhaps see precedence for union with Christ here too. Paul may be making this equation when he talks about the Church being the bride of Christ.
 See Peterson, Salvation Applied by the Spirit, 234-48.
 “In the case of both Adam and Christ, one action has had consequences of universal significance” (Philip Edgcumbe Hughes, Paul’s Second Epistle to the Corinthians [Grand Rapids: Wm. B. Eerdmans Publishing Co., 1962], 195)
 “The idea that the destinies of the Messiah and the people of God are linked is not unique (e.g. Dan. 7:9-27; 2 Baruch 30)” (M. A. Seifrid, “In Christ” in Dictionary of Paul and His Letters ed. Gerald F. Hawthorne, Ralph P. Martin, and Daniel G. Reid (Downers Grove: InterVarsity Press, 1993), 435).