However, we have St. Paul himself engaging in a similar expression of gratitude in his letter to the Colossians (1:24): "Now I rejoice in my sufferings for your sake, and in my flesh I complete what is lacking in Christ's afflictions for the sake of his body, that is, the Church," -- a verse which I personally found very challenging. If Christ's redemptive act on the cross is final, it is understandable why anyone would be confused by Paul's expression.
Exploring this topic further, I have been reading the Holy Father's letter "On The Christian Meaning of Human Suffering", an extensive and exceptional meditation on this topic. Part V addresses the manner in which the meaning of human suffering has been transformed in light of the Cross. In section 24, he provides an interpretation of St. Paul's verse:
For, whoever suffers in union with Christ -- just as the Apostle Paul bears his "tribulations" in union with Christ -- not only receives from Christ that strength already referred to but also "completes" by his suffering "what is lacking in Christ's afflictions". This evangelical outlook especially highlights the truth concerning the creative character of suffering. The sufferings of Christ created the good of the world's redemption. This good in itself is inexhaustible and infinite. No man can add anything to it. But at the same time, in the mystery of the Church as his Body, Christ has in a sense opened his own redemptive suffering to all human suffering. In so far as man becomes a sharer in Christ's sufferings -- in any part of the world and at any time in history -- to that extent he in his own way completes the suffering through which Christ accomplished the Redemption of the world.The Pope's description of the redemptive act being "unceasingly completed" is challenging, and I would recommend his entire letter for a fuller explication. But it is clear from the writings of the fathers and the lives of the saints, that they approach their own personal sufferings as an opportunity to share in the passion of Christ on the Cross, the redemptive value of which transforms human suffering and even bursts the confines of time itself. I think that is what Paul Claudel means in his example of a shipwreck survivor, bound tightly to the mast and floating over an open sea, finding in his physical experience the opportunity to join in the sufferings of his Savior.
Does this mean that the Redemption achieved by Christ is not complete? No. It only means that the Redemption, accomplished through satisfactory love, remains always open to all love expressed in human suffering. In this dimension -- the dimension of love -- the Redemption which has already been completely accomplished is, in a certain sense, constantly being accomplished. Christ achieved the Redemption completely and to the very limits but at the same time he did not bring it to a close. In this redemptive suffering, through which the Redemption of the world was accomplished, Christ opened himself from the beginning to every human suffering and constantly does so. Yes, it seems to be part of the very essence of Christ's redemptive suffering that this suffering requires to be unceasingly completed.
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Like other religious commentaries on suffering, the Pope uses the Book of Job as a chief example. The experience of Job, the Pope says, demonstrates that suffering cannot necessarily be interpreted in moral terms, as "divine punishment" for a crime:
While it is true that suffering has a meaning as punishment, when it is connected with a fault, it is not true that all suffering is a consequence of a fault and has the nature of a punishment. The figure of the just man Job is a special proof of this in the Old Testament. Revelation, which is the word of God himself, with complete frankness presents the problem of the suffering of an innocent man: suffering without guilt. Job has not been punished, there was no reason for inflicting a punishment on him, even if he has been subjected to a grievous trial. . . . The Book of Job is not the last word on this subject in Revelation. In a certain way it is a foretelling of the Passion of Christ. But already in itself it is sufficient argument why the answer to the question about the meaning of suffering is not to be unreservedly linked to the moral order, based on justice alone.In the end Job's friends, who had tried to justify his suffering, are reproved by God, and He recognizes that Job did not deserve the tragedies which befell him. "His suffering is the suffering of someone who is innocent and it must be accepted as a mystery, which the individual is unable to penetrate completely by his own intelligence." In situations like Job's, where the rightous and innocent are subject to unnecessary suffering, to interpet suffering in a moral framework (as Job's friends did) "seems to trivialize and impoverish the concept of justice which we encounter in Revelation."
We don't necessarily have an answer as to why suffering occurs sometimes, nor can we hope to arrive at a satisfactory explanation, given our limited human perspective. But as the Pope explains, Christ has imbued the experience of human suffering with the potential for spiritual transformation, just as God the Father used the passion of His Son to conquer death and reconcile man to Himself. What Jesus' disciples initially experienced as a crushing defeat, the unjust persecution and execution of their rabbi and messiah, became instead the constitutive act of God's saving love.
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In her article, "To Love is to Suffer," (Canticle, Issue 11, Winter 2001), Dr. Lin Wilder draws our attention to the experience of the Dr. Victor Frankl, who spent 3 years in Aushwitz, Dachau, and other Nazi concentration camps (one of the most vivid examples of human suffering in modern times). Dr. Frankl's book Man’s Search For Meaning chronicles how the memory of his wife (separated from him in another camp) leads to the epiphany which made his suffering endurable:
. . . A thought transfixed me: for the first time in my life I saw the truth as it is set into song by so many poets, proclaimed as the final wisdom by so many thinkers. The truth - that love is the ultimate and the highest goal to which man can aspire. Then I grasped the meaning of the greatest secret that human poetry and human thought and belief have to impart: The salvation of man is through love and in love. I understood how a man who has nothing left in this world still may know bliss, be it only for a brief moment, in the contemplation of his beloved. In a position of utter desolation, when man cannot express himself in positive action, when his only achievement may consist in enduring his sufferings in the right way - an honorable way - in such a position man can, through loving contemplation of the image he carries of his beloved, achieve fulfillment. For the first time in my life I was able to understand the meaning of the words, "The angels are lost in perpetual contemplation of an infinite glory."