In medieval theology, union with Christ was not a fixed reality; it was something that could fluctuate and change over one’s spiritual pilgrimage. It was the believer’s responsibility, therefore, to seek greater and more complete union with Christ through prayer, the sacraments, obedience, and so on.
The Reformers, however, believed the Scriptures made a distinction between union with Christ and communion with Christ.
On the one hand, union with Christ is fixed upon our first act of saving faith in Jesus. At that moment of faith, we are united to Jesus legally as our covenant head (Rom 3:26; 5:12-21) and we are united with him spiritually as the source of all our spiritual life (John 15:1ff). Our union with Christ—like justification—cannot grow or change. It is always fixed, solid, unchanging. On the other hand, our communion with Christ—that is, our actual enjoyment of Christ—ebbs and flows during the course of our lives. Our affections are one day fervent and bright; the next day they are cool and dull.
As you can see, it is vital to maintain this distinction between union with Christ and communion with Christ, for in order to have genuine enjoyment of Christ in communion with him, our relationship with Christ must be firm and fixed, and our righteous standing before God must be found in something other than our inconsistent works and affections. How can we enjoy Christ when we are unsure whether or not our deeds and prayers have brought us into greater union with him?
In the medieval scheme, it was the person’s works, their diligence in seeking God, and their spiritual affections that achieved greater union with Christ. That is, it was subjective effort grounding subjective experience. In Reformation theology, it is an unchanging union with Christ that leads unfailingly to works of sanctification, diligence in seeking God, prayer, and affections for Christ. In the latter case, it is the objective status grounding the subjective experience. Because the objective status doesn’t change and is unaffected by the subjective experience (e.g., works, prayers, affections), the believer is actually supplied with an infinitely more reliable foundation upon which to build his or her enjoyment of Christ (i.e., their subjective experience).
Although it may appear that proponents of this medieval scheme valued communion with Christ, the basis on which they could have a robust experience of Christ was actually obliterated by this misunderstanding of union with Christ. But Scripture teaches that when we are united to Christ, we are no longer liable to the punishment of death because we were already punished in the death of Christ, and we now enjoy newness of life because we are united to Christ in his resurrection (Rom 6:1-4; Gal 2:20).
We possess Christ’s righteousness, not because he hands over some commodity to us, but because we are united to him. In other words, we possess Christ’s righteousness because we possess him. Or even better, because he possesses us (John 10:27-30; Rom 8:39; Phil 3:12).