Colossians 1:24

Who--The oldest manuscripts omit "who"; then translate, "Now I rejoice." Some very old manuscripts, and the best of the Latin versions, and Vulgate, read as English Version. To enhance the glory of Christ as paramount to all, he mentions his own sufferings for the Church of Christ. "Now" stands in contrast to "I was made," in the past time (Col 1:23). for you--"on your behalf," that ye may be confirmed in resting solely on Christ (to the exclusion of angel-worship) by the glorification of Christ in my sufferings (Eph 3:1). fill up that which is behind--literally, "the deficiencies"--all that are lacking of the afflictions of Christ (compare Note, see on Co2 1:5). Christ is "afflicted in all His people's afflictions" (Isa 63:9). "The Church is His body in which He is, dwells, lives, and therefore also suffers" [VITRINGA]. Christ was destined to endure certain afflictions in this figurative body, as well as in His literal; these were "that which is behind of the afflictions of Christ," which Paul "filled up." His own meritorious sufferings in expiation for sin were once for all completely filled up on the Cross. But His Church (His second Self) has her whole measure of afflictions fixed. The more Paul, a member, endured, the less remain for the rest of the Church to endure; the communion of saints thus giving them an interest in his sufferings.

— Jamieson-Fausset-Brown Commentary (public domain)