Colossians 1:24 - Wesley's Explanatory Notes

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.