The doctrine of Union with Christ is central to the Biblical doctrine of salvation. Therefore, with extreme brevity, I intend to explain aspects of union with Christ, that you might delve more deeply into the Scriptures and the treasures of Reformed Theology.
Union with Christ in Eternity: Union of Election
Election is that act of God whereby he chooses some from the mass of mankind to be recipients of his salvation in Christ. This union is, in one sense, the union of necessary realization. Though united to Christ in election, all mankind is born “in Adam.” The elect are likewise born children of wrath. Yet, due to God’s purpose in election, they are given the gift of salvation at some point in their lives (Ephesians 1:4).
Union with Christ in History: Union of Redemptive-History
The redemptive historical union is that union realized in the incarnation, wherein Christ took upon himself our human nature and became unto us the perfect representative in our place. He fulfilled God’s law as our representative and, most intimately, our sin was united to Him on the cross. “He became sin who knew no sin that we might become the righteousness of God” (2 Corinthians 5:21). This union is redemption accomplished.
Union with Christ in Personal Experience: The Existential Union
This union is the union of personal experience. Through the existential union, all the benefits of salvation are conveyed to the person. These benefits include regeneration, justification, adoption, sanctification, ect. All the benefits of salvation applied are rooted and grounded in our union with Christ by the Holy Spirit.
Union with Christ in the Church: The Covenantal Union
Though none who are united to Christ, mystically and salvifically, can fall away, there is a type of union with Christ’s body in Scripture which is terminable. This is the union of persons to the institutional church as the body of Christ. In short, you can be united to Christ in the sense of being united to his external body, the church, and not be united to him vitally (John 15:1-11, Romans 11:16-24, 1 John 2:19, Hebrews 10:26-31).
Union with Christ Forever: The Eschatological Union
Eschatology is the study of the end things. Union with Christ pertains to Eschatology, and the Christian believer, because it is in the consummation of the age that the believer realizes the most intimate union with Christ, that of being with Him for eternity (Colossians 3:4).
For More on the Doctrine of Union with Christ see:
A Puritan Theology: The Puritans on Union with Christ, Justification, and Regeneration, Mark Jones and Joel Beeke, Pgs 481-491.