This is the text in the encyclical Redemptor Hominis that connects with this question how Christ united Himself with all human beings: “We are dealing with “each” man, for each one is included in the mystery of the Redemption and with each one Christ has united himself for ever through this mystery.” (Redemptor Hominis, Section 13). One of the concepts that has impressed me in this encyclical is that the Incarnation linked Jesus to all mankind, that he is connected to us through our mutual humanity. In becoming human He became one of us, united with us in the weakness and humility of the human body. In some ways this seems self-evident, yet I find it powerful to reflect on the commonality I feel with Jesus in his human body and bodily functions.
The more difficult idea, I think, is that through the Redemption Jesus is united to us. This is hard because it is tempting to picture the Redemption as something that Jesus did “for” us, for me. This notion leaves me with the image of separateness, with Jesus “out there” doing something for me “in here” as I cannot connect with the immensity and transcendence of His passion, death, and resurrection. I had been taught about “offering up” suffering, though this is, again, so insignificant compared to the staggering level of efficacy and suffering by Jesus in his passion and death by crucifixion.
In his great genius John Paul II demonstrates the uniting of Christ with all human beings in the Redemption through human suffering, as described in his Apostolic Letter, Salvifici Dolores. In it he explains: “In the Cross of Christ not only is the Redemption accomplished through suffering, but also human suffering itself has been redeemed. Christ, - without any fault of his own - took on himself 'the total evil of sin'. The experience of this evil determined the incomparable extent of Christ's suffering, which became the price of the Redemption.” (Salvifici Dolores, Section 19). Thus, not only is Jesus united with us bodily in His physical humanity, he is united with us psychologically and spiritually in human suffering—he united with us in and within our experience of evil.
And, by redeeming our human suffering, he frees us to suffer voluntarily with Him, in His redemptive suffering, and, thus, enter into salvation, into theosis with Him through his Resurrection and Ascension. As John Paul II so brilliantly writes: “This interior maturity and spiritual greatness in suffering are certainly the result of a particular conversion and cooperation with the grace of the Crucified Redeemer. It is he himself who acts at the heart of human sufferings through his Spirit of truth, through the consoling Spirit. It is he who transforms, in a certain sense, the very substance of the spiritual life, indicating for the person who suffers a place close to himself. It is he—as the interior Master and Guide—who reveals to the suffering brother and sister this wonderful interchange, situated at the very heart of the mystery of the Redemption. Suffering is, in itself, an experience of evil. But Christ has made suffering the firmest basis of the definitive good, namely the good of eternal salvation. By his suffering on the Cross, Christ reached the very roots of evil, of sin and death. He conquered the author of evil, Satan, and his permanent rebellion against the Creator. To the suffering brother or sister Christ discloses and gradually reveals the horizons of the Kingdom of God: the horizons of a world converted to the Creator, of a world free from sin, a world being built on the saving power of love. And slowly but effectively, Christ leads into this world, into this Kingdom of the Father, suffering man, in a certain sense through the very heart of his suffering. For suffering cannot be transformed and changed by a grace from outside, but from within. And Christ through his own salvific suffering is very much present in every human suffering, and can act from within that suffering by the powers of his Spirit of truth, his consoling Spirit.(Savifici Dolores, Section 25)”