Colossians 1:24 says:
24Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ’s afflictions for the sake of his body, that is, the church, 25 of which I became a minister according to the stewardship from God that was given to me for you, to make the word of God fully known, 26 the mystery hidden for ages and generations but now revealed to his saints.
The commission to reveal to the Gentiles the message of the gospel has been given to Paul, and Jesus never fulfilled such a role. What is lacking in Christ’s afflictions is that the benefit of those afflictions had not yet been proclaimed among the Gentiles. In salvation history the unique privilege of bringing the good news of Christ to the Gentiles belongs to Paul. And like Christ, Paul heralds a message advanced in and through his suffering. … Paul’s sufferings mirror and reflect what Christ has done, so that the messenger in this sense replicates the life of the one proclaimed.
It is significant that suffering is the means by which Paul extends the message to Gentiles. Such suffering highlights the weakness of the messenger and points hearers to God as the all-sufficient one. God’s work in and through a weak and suffering Paul indicates that the power comes from God rather than Paul. The age to come has invaded this present evil age since God reveals his life in earthen vessels. Suffering also validates the integrity of Paul as a messenger, for his willingness to undergo pain indicates that he does not proclaim the gospel as a way to get rich or to promote his reputation on earth. Paul’s sufferings are a corollary to the sufferings of Christ inasmuch as they are the means by which the message is brought to the Gentiles. It is not the case that God desired Paul to bring the message to the Gentiles and afflictions got in the way. Suffering was the intended means by which the gospel would be proclaimed by Paul to the Gentiles. Thereby the focus remains on God and Christ, and glory does not redound to the proclaimer but to the proclaimed.
Quoted from Schreiner, T. R. (2001). Paul, Apostle of God’s glory in Christ: A Pauline theology. Downers Grove, Ill: InterVarsity Press, p. 102