The concept of a believer’s union with Christ refers both to a positional definition and an experiential definition. Positionally, when God looks down at us He doesn’t see the times we’ve messed up or the sins we’ve committed. We God looks down at a Christian believer, He sees the the acts that Jesus did and the life that Jesus lived. Because of this, that means positionally before God we get to take the place that Jesus gets. Experientially, God has come down to earth and He lives inside of every Christian believer. We don’t just cognitively know of God, we know God through our experiences with Him. Through the Holy Spirit living within believers, God begins the process of Sanctification within our lives leading us through a long process to becoming more and more like Jesus.
Without the union of Christ, the entire Christian doctrine of Salvation would fall apart. This is because the concept of our union with Christ is embedded in 4 crucial aspects of our salvation: eternally, historically, presently and eschatologically. Eternally, God chose us in Christ. Historically, God sees us as united with Christ in His life, death and resurrection. Presently, our salvation and the gift of the Holy Spirit inside of us is only because we are united with Christ. Eschatologically, when a believer dies they are “with Christ” and will rule with Christ (Rev 3:21).