While union with Christ is not a specific phrase found in Scripture, the doctrine presents itself in many other grammatical ways. For example, in Christ is found over ninety times in the New Testament. First Corinthians 15:22 is one example: “For as in Adam all die, so also in Christ all will be made alive.”
Additionally, the phrases in him and in whom often represent union with Christ. For example, Ephesians 1:7 says, “In him we have redemption through his blood, the forgiveness of sins, in accordance with the riches of God’s grace,” and Colossians 1:13–14 states, “He has rescued us from the dominion of darkness and brought us into the kingdom of His beloved Son, in whom we have redemption, the forgiveness of sins.”
Scripture uses metaphors that further support the reality of our union with Christ: Jesus is the Vine, and we are the branches (John 15:5). Jesus is the Head, and we are His body (Colossians 1:18). Jesus is the Cornerstone, and we are the living stones built upon Him (1 Peter 2:4–7). He is the Bridegroom, and we are His bride (Ephesians 5:31–32; Revelation 19:7–9). We are crucified with Christ (Galatians 2:20) and baptized into Him (Romans 6:3).
Pastor and theologian John Piper defines union with Christ this way: “The reality of all the ways that the Bible pictures our human connectedness to Christ, in which He is indispensable for every good that we enjoy” (“What Is Union with Christ?” interview, www.desiringgod.org, accessed 5-13-20). These good riches include the following:
• being chosen by God and given new life (Ephesians 1:4; 2:4–5, 10)
• justification (1 Corinthians 1:30; 2 Corinthians 5:21; Philippians 3:8–9)
• sanctification: having a new life that obeys God (Galatians 2:20; Ephesians 3:16–17; 4:16; 1 Corinthians 1:30; John 15:4–5; 2 Corinthians 5:17)
• a new identity in this life and the next (Romans 14:8; 1 Thessalonians 4:16; Revelation 14:13)
• the indwelling of the Holy Spirit, the Comforter (John 14:16–17; 1 Corinthians 3:16)
• preservation/perseverance in the faith (John 10:27–28; Romans 8:38–39)
• bodily resurrection in the future (Colossians 3:1; 1 Corinthians 15:22)
• glorification in the future (Colossians 3:4; 1 Thessalonians 4:16–17)
Union with Christ is the essence of what it means to be a Christian. When considering our salvation, our mystical union with the Lord should be given “the highest degree of importance,” according to Calvin. “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” (Institutes of the Christian Religion, Donald McKim, ed., Westminster John Knox Press, 2001, III.11.10).