John Paul 2 on the Redemptive Value of Suffering

"Saint Paul speaks of such joy in the Letter to the Colossians: "I rejoice in my sufferings for your sake"(88). A source of joy is found in the overcoming of the sense of the uselessness of suffering, a feeling that is sometimes very strongly rooted in human suffering. This feeling not only consumes the person interiorly, but seems to make him a burden to others. The person feels condemned to receive help and assistance from others, and at the same time seems useless to himself. The discovery of the salvific meaning of suffering in union with Christ transforms this depressing feeling. Faith in sharing in the suffering of Christ brings with it the interior certainty that the suffering person "completes what is lacking in Christ's afflictions"; the certainty that in the spiritual dimension of the work of Redemption he is serving, like Christ, the salvation of his brothers and sisters. Therefore he is carrying out an irreplaceable service. In the Body of Christ, which is ceaselessly born of the Cross of the Redeemer, it is precisely suffering permeated by the spirit of Christ's sacrifice that is the irreplaceable mediator and author of the good things which are indispensable for the world's salvation.
It is suffering, more than anything else, which clears the way for the grace which transforms human souls. Suffering, more than anything else, makes present in the history of humanity the powers of the Redemption. In that "cosmic" struggle between the spiritual powers of good and evil, spoken of in the Letter to the Ephesians(89), human sufferings, united to the redemptive suffering of Christ, constitute a special support for the powers of good, and open the way to the victory of these salvific powers. "

From the Apostolic Letter Salvifici Doloris 2/11/1984