Colossians 1:24

Colossians 1:24. And fill up that which is behind of the afflictions of Christ in my flesh for his body’s sake, which is the church:

There is nothing "behind" as to the atoning efficacy of the sufferings of Christ, but there is much yet to be endured in order that all the elect may be brought to Christ. Some must suffer through their extraordinary labours in preaching the gospel, others through bearing reproach for the truth’s sake, and Paul was glad to take, in his mortal body, his share of the sufferings to be endured for the sake of Christ’s Church, which is his mystical body.