Robert Letham briefly summarizes the connection between union with Christ and our justification, sanctification, and resurrection:
Union and Justification
According to Paul in Romans 5:12-21, just as Adam plunged the whole race into sin and death because of their relationship of solidarity with him, so the second Adam brings life and righteousness to all who sustain a relationship of solidarity with him
If, because of one man’s trespass, death reigned through that one man, much more will those who receive the abundance of gracee and the free gift of righteousness reign in life through the one man Jesus Christ. (Rom. 5:17 ESV)
Here Paul reflects on his previous statement of the one way of salvation from sin by the propitiatory death of Christ, which avails for all who believe (Rom. 3:21ff). Justification is received only by faith and is grounded in what Christ did once for all in his death and resurrection (4:25). Paul’s point is that we are not addressed merely as discrete individuals; instead, we are a team of which we all were members. His sin plunged the whole team into sin, ruin, death, and condemnation. What Christ did for us was also done as the head of a team of which we are a part. He did it on our behalf, for us–and God reckons it to our account as a result of our being united, through faith, with him as the head of the team. Our justification is therefore grounded on union with Christ.
Union and Sanctification
In Romans 6:1ff, in answer to charges that his gospel encourages moral indifference, Paul insists that believers, the justified, live to Christ and do not give themselves over to sin. This is because they died with Christ to sin and rose again to new life in his resurrection. Not only did Christ die and rise again for them, but they died and rose with him. Union with Christ is the foundational basis for sanctification and the dynamic force that empowers it. As Paul says, “Do you not know that as many were baptized into Christ Jesus were baptized into his death; we were buried with him through baptism into death, so that as Christ was raised from the dead through the glory of the Father so we too should live in newness of life” (6:3-4).
Union and Resurrection
Paul argues in 1 Corinthians 15 that the resurrection of Christ and the future resurrection of his church is one reality (vv. 12-19). Paul argues back and forth from one to the other. If Christ is not raised, there can be no resurrection of believers. If there is no general resurrection, Christ cannot have been raised himself. The two stand together. In fact, Christ has been raised–and so, therefore, will we be. Christ is the firstfruits of the resurrection of believers at his return (vv. 19-23). Not only is his resurrection first in time, but as firstfruits, it is of the same kind as the full harvest. Hence, it is the guarantee not only that the full harvest will be gathered but that both his resurrection and ours are identical. From this it is clear that the resurrection of believers at the parousia is a resurrection in Christ. The resurrections are effectively the same…Christ resurrection and the resurrection of the righteous, separated by indefinite time, are identical because the later occurs in union with the former.
Union with Christ: In Scripture, History, and Theology, pg. 5-7
Each of those points could be expanded upon at length, but this brief summary gives us a glimpse into the way the biblical record places our union with Christ at the blazing center of our salvation through Christ. There is no Gospel without union.
Soli Delo Gloria