“Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:”
King James Version (KJV)
1:24 Now I rejoice in my sufferings for you, and fill up - That is, whereby I fill up. That which is behind of the sufferings of Christ - That which remains to be suffered by his members. These are termed the sufferings of Christ, Because the suffering of any member is the suffering of the whole; and of the head especially, which supplies strength, spirits, sense, and motion to all. Because they are for his sake, for the testimony of his truth. And these also are necessary for the church; not to reconcile it to God, or satisfy for sin, (for that Christ did perfectly,) but for example to others, perfecting of the saints, and increasing their reward.
Col 1:24 Who now rejoice. Rather, I now rejoice. My sufferings for you. He was a prisoner and a sufferer because he preached the gospel of the Gentiles. And fill up that which is behind of the afflictions of Christ. Christ endured afflictions for us. We must have a fellowship of his sufferings (1Pe 4:13). Paul also suffers that he might share the afflictions of Christ. Not only did he suffer with Christ, but Christ suffers with his afflicted people. See Ac 9:4. Hence, afflictions suffered for Christ may be called his afflictions. All the suffering required for the sake of the church is "behind of the afflictions of Christ".