Charles Spurgeon Commentary Colossians 1:24

"Now I rejoice in my sufferings for your sake, and fill up on my part that which is lacking of the afflictions of Christ in my flesh for his body`s sake, which is the church;" — Colossians 1:24 (ASV)

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" regarding the atoning efficacy of the sufferings of Christ, but much is still to be endured so 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 sake of the truth. 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.

Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:

As if there were so much suffering to be endured to bring in the redeemed from the world, and so much self-sacrifice to be made in order that those whom Christ has redeemed may come to know of that redemption, and may be brought to him, and Paul was glad to make up that which was behind of the afflictions of Christ in his flesh, for his body's sake, which is the church:.