Commentaries:
The Amplified Bible renders this:
[Even] now I rejoice in the midst of my sufferings on your behalf. And in my own person I am making up whatever is still lacking and remains to be completed of Christ's afflictions, for the sake of His body, which is the church.
Paul claims that he is in the process of completing or filling up what remained for his afflictions to get to the level of Jesus Christ's afflictions. What does he mean? The Greek word translated as "afflictions" is never used in reference to the crucifixion. Instead, it refers to the pressure, the troubles, and the distress Christ suffered during His life. The apostle is saying that all the afflictions that Jesus suffered are at one level, and all of his own afflictions are at a lower one, but by laboring on behalf of the Gentiles in Colossae, Paul was filling up the difference between them.
This expresses Paul's greatest desire to be just like Christ, including having the same experiences His Savior did. Paul was suffering in the cause of the church, just as Christ did. Paul was enduring the rejection, the contempt, and the abuse, similar to what Christ did. Paul was not suggesting that he would ever complete all that was lacking, only that as he was suffering, he could compare it with Christ's suffering and say, "I'm not there yet, but I'm getting closer." Paul yearned so greatly to be made just like Christ in all respects that he rejoiced at the opportunity to experience more of what Christ endured and labor on behalf of the Body of believers just as Christ did.
David C. Grabbe
A Look at Christian Suffering (Part Two)
Paul is writing to Gentile converts, and the specific mystery that he is talking about here—our hope—is Christ in us. It was no secret in the Old Testament that the Gentiles would be saved, but that Christ, by means of the Holy Spirit, would dwell in all converts was unknown. Otherwise, Paul would be telling a lie when he said that it is a mystery now revealed to the church. This applies directly to us because it is Christ living His life in us that prepares us for the Kingdom of God. God in us is our hope!
This hope gives us certainty for the future, which carries beyond the grave, even as it did with Christ. He rose from the grave because God was in Him. This hope is why we can have joyful and confident expectation of salvation because Christ's life—His character, values, virtues, thoughts, attitudes, and deeds—can become evident in a Christian.
John W. Ritenbaugh
Perseverance and Hope