I’m sure this has been helpfully explained before, but a light came on a couple weeks back in understanding what Paul is getting at in Col 1.24 where he writes, ‘Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ’s afflictions’.
The question is, is something about Christ’s work on the cross insufficient? The answer, of course, is ‘no’. But what does Paul mean here?
I think the easiest and most helpful way to understand this is to think about the one inflicting the suffering. On the cross it was the wrath of God against the sin of his people that Jesus suffered. The suffering Paul experiences (and is lacking in Christ) does not come from the Father to top-up what was insufficient in the cross, but comes from the enemy.
You might say it this way, in Christ’s death on the cross the wrath of God was satisfied, but not the wrath of Satan (not that Satan was pouring wrath on Jesus on the cross, he wasn’t; but even in his observance of these sufferings of Christ, he wants to see more).
So, the enemy is not done inflicting sufferings upon Christ’s body, and Paul was filling up what was lacking and now the church continues to fill up what is lacking.