Paul says a peculiar thing in Colossians. "I rejoice in my suffering for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of his body, the church..." (Colossians 1:24). How can anything be lacking in the sufferings of Christ?
It is a problem of translating across cultures. Our modern minds put modern meanings to his words. He is not saying that the cross is not sufficient to cover sin and he must somehow make up the difference. Nor is he saying Christ did not finish the work his Father sent him to do and Paul must finish it for him.
What he is saying is that Christ left us work to do, preach the gospel (Mt 28:18-20) and that task involves suffering at times (John 16:1-4, 33). Fulfilling what is lacking in the afflictions of Christ means there is suffering associated with spreading the gospel and there is more suffering to be done because not everyone has heard yet.
It is like saying, our work in spreading the gospel is not finished yet; we are lacking people who must hear and that will involve suffering because we are preaching truth. When we preach we are sharing in the sufferings of Christ. He has granted to us this gift of sharing in everything including his inheritance (Romans 8:17).
