Now I rejoice in my sufferings for your sake, and in my flesh I do my share on behalf of His body, which is the church, in filling up what is lacking in Christ's afflictions.Colossians 1:24(NASB)
Paul is able to rejoice in his sufferings because he was counted worthy to suffer for the sake of Jesus Christ. Like Paul we too can know Christ and the power of His resurrection. We too can fellowship in His suffering - and fill up what is lacking in Christ's afflictions.
Participating in Christ's sufferings has nothing to do with His atoning sacrifice. Only the sinless Lamb of God could die for the sins of the world and He died once and for all upon the cross. His death at Calvary was to redeem mankind from their sins, and we have no part in His redeeming sacrifice, for we are saved by grace through faith.
But Christ is the head of the Church, and as members of His body. He feels the same sufferings and pain that we have to go through in life.. and when we share in the afflictions of our brothers and sisters in Christ, in a small way we also help to fill up what is lacking in Christ's afflictions - and the more we suffer for Christ, the more God will shower us with His comfort through Christ Jesus our Lord.
Loving heavenly Father, as I share the pressure, pain, problems and persecutions of my fellow brothers and sisters in Christ, may I rejoice in my sufferings, knowing that in so doing I am helping to fill up the afflictions of Christ - to Your praise and glory. Thank You in Jesus name. AMEN.