“I now rejoice in my sufferings for you, and fill up in my flesh what is lacking in the afflictions of Christ, for the sake of His body, which is the church…” Colossians 1:24.
The most important thing to grasp about this especially difficult passage is that it has absolutely nothing to do with the sufferings of Christ on the cross. There was nothing left undone in the Lord’s sacrifice for sin; meaning that the apostle is not here speaking of something which was not covered on the cross.
“But this Man, after He had offered one sacrifice for sins forever, sat down at the right hand of God, from that time waiting till His enemies are made His footstool. For by one offering He has perfected forever those who are being sanctified.” Hebrews 10:12-14.
The Phillips Version of the New Testament actually translates our verse in Colossians in the clearest manner of the several versions available.
“I myself have been made a minister of this same gospel, and though it is true at this moment that I am suffering on behalf of you who have heard the gospel, yet I am far from sorry about it. Indeed, I am glad, because it gives me a chance to complete in my own sufferings something of the untold pains which Christ suffers on behalf of his body, the Church.” Colossians 1: 24 J.B. Phillips New Testament in Modern English.
Paul’s rejoicing in his sufferings for the Colossians, and by extension, the church which he was establishing, was not some dysfunctional masochistic thing. There are many today who live under the assumption that if they are sickly, in abject poverty, or are continually enduring one trial after another that they are suffering for Christ.