What does Colossians 1:24 mean?

After mentioning his work as a minister of the gospel in the previous verse, Paul now includes further detail about his efforts (Colossians 1:24–29). This verse emphasizes Paul's sufferings. Writing from Roman house arrest, he declares that he can still rejoice, even though he is under hardship. Just as James taught (James 1:2–4), Paul could rejoice in suffering because he knew that the end result was glory in God. Paul suffered both for Christ, and on behalf of Paul's fellow believers.

The way Paul refers to Christ's afflictions here has been the subject of much controversy. While there are many views on this topic, the most likely is that Paul viewed his suffering as a "service," while Christ's was a "sacrifice." The church, in terms of a group of believers, did not yet exist when Christ suffered on the cross. Now, however, Paul suffered for the church as part of his ministry. The term Paul uses for "afflictions" is thlipseōn, from the root word thlipsis, which is not a term the New Testament uses in reference to the physical sufferings of Christ. While Christ certainly suffers when His church suffers (Acts 9:4), Paul's experiences are a different kind of trouble, meant for a different purpose than the sacrificial death of Christ.

In other words, Paul is not suggesting that his suffering makes up something lacking in the saving power of Christ's death. Rather, he considers his persecution a service, one which Christ left for His followers to fulfill.