In Colossians 1:24, I read:
Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ’s afflictions for the sake of his body, that is, the church (ESV)
I'm very confused by this verse. Does Paul here consider Christ's afflictions lacking in some way? It seems so, but in what way? And how does Paul's own suffering make up for this lacking?
I think it is important that he doesn't say something like "what is lacking in the atonement of Christ" or even "in the cross of Christ" but "what is lacking in the suffering of Christ." Here's the most relevant portion of what Calvin says on this verse:
We know that there is so great a unity between Christ and his members, that the name of Christ sometimes includes the whole body, as in 1 Corinthians 12:12, for while discoursing there respecting the Church, he comes at length to the conclusion, that in Christ the same thing holds as in the human body. As, therefore, Christ has suffered once in his own person, so he suffers daily in his members, and in this way there are filled up those sufferings which the Father hath appointed for his body by his decree. Here we have a second consideration, which ought to bear up our minds and comfort them in afflictions, that it is thus fixed and determined by the providence of God, that we must be conformed to Christ in the endurance of the cross, and that the fellowship that we have with him extends to this also.
He adds, also, a third reason—that his sufferings are advantageous, and that not merely to a few, but to the whole Church. He had previously stated that he suffered in behalf of the Colossians, and he now declares still farther, that the advantage extends to the whole Church. This advantage has been spoken of in Philippians 1:12. What could be clearer, less forced, or more simple, than this exposition, that Paul is joyful in persecution, because he considers, in accordance with what he writes elsewhere, that we must carry about with us in our body the mortification of Christ, that his life may be manifested in us? (2 Corinthians 4:10)