en_tn_lite_do_not_use/col/01/24.md

I fill up in my flesh what is lacking of the afflictions of Christ

Paul speaks about the suffering that he continues to experience. He may be acknowledging here that there is much suffering that he and all other Christians must endure before Christ comes again, and that Christ in a spiritual sense joins with them in experiencing these hardships. Paul certainly does not mean that Christ's sufferings alone were not enough to provide salvation for the believers.

I fill up in my flesh

Paul speaks of his body as if it were a container that could hold suffering. (See: rc://en/ta/man/jit/figs-metaphor)

for the sake of his body, which is the church

Paul often speaks of the church, the group of all Christian believers, as if it were Christ's body. (See: rc://en/ta/man/jit/figs-metaphor)