Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in the afflictions of Christ on behalf of his body, which is the church ...
Pope Saint John Paul II explained how individuals can share in the sufferings of Christ:
"...Christ achieved the Redemption completely and to the very limits but at the same time he did not bring it to a close....every form of suffering, given fresh life by the power of this Cross, should become no longer the weakness of man but the power of God."
"By embracing the many crosses of daily life, a person not only sanctifies himself, but also releases a flood of graces for others. This is what Paul meant by completing what is lacking in Christ's afflictions for the sake of the Church. The body of Christ is not merely a collection of Christians. It is a living instrument of redemption - an extension of Jesus Christ throughout time and space. He continues his salvific work through each member of his body. When a person understands this, he sees the idea of "offering it up" is not just a theological reply to the question of human suffering, but rather a call to participate in the salvation of the world.
From Saint John Paul the Great His Five Loves by Jason Evert.