“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/salvation/saved (1 Corinthians 1:30; 2 Corinthians 5:21; Philippians 3:8–9)

• sanctification: having a new life that obeys God after salvation (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(don’t quit) 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.”

https://www.gotquestions.org/union-with-Christ.html