Someone recently asked me what Paul meant when he wrote to the Colossian believers, “I Paul am made a minister, who now rejoice in my sufferings for you, and fill up that which is behind (lacking) of the afflictions of Christ in my flesh for his body’s’ sake, which is the church.” (Col 1:23-24). What did Paul mean by “fill up that which is behind of the afflictions of Christ in my flesh?” A literal translation reads, “And in my flesh I complete (fill up) what is lacking in Christ’s afflictions for the sake of his body, the church.” This is a much-disputed verse, but I believe the general sense of it can be made clear.
Let it be said, Paul was by no means inferring that Christ’s suffering and death was not sufficient payment for our sin and that he or any other saint must do something to “fill up” or bring to completion the atoning work of Christ. He is not saying there was anything deficient in the sufferings which Christ endured to atone for our sins. By no means does this verse teach Christ’s atonement is defective and that the sufferings of the saints in some way are necessary to supplement His work on our behalf. Nothing in Paul’s writings ever contradicted the Apostle’s belief that Christ’s death was absolutely totally sufficient and complete to atone for sin.
So what did Paul mean when he said that something in Christ’s afflictions was “still lacking?” The Greek word translated “fill up” or “complete” in Colossians 1:24 occurs only here in the NT. It is the word antanaplēróō which is made of two words that mean “corresponding to” or “in place of” and “fulfill” or “complete;” meaning to “fill up or complete what is wanting or lacking.” Paul, and the early Christians, recognized that suffering or persecution for the cause of Christ was an accepted reality. For Paul, union with Christ involved union with Christ’s sufferings (Phil. 3:10), and he was willing to suffer for the sake of the Body of Christ, the Church. There is nothing “behind or lacking” as to the atoning efficacy of the sufferings of Christ, but there is much yet to be endured by His followers for the further spread of the Gospel and the building-up of His Body, the Church. Paul’s sufferings were for the Church’s sake and the furtherance of the Gospel. Paul in enduring persecution for the Gospel’s sake, which corresponded to sufferings as Christ endured from persecutors during HIs earthly ministry , was for the purpose of advancing the message of Christ’s atoning work on the cross. The enemies of Christ when persecuting Paul were in reality persecuting Christ. Whatever the church suffers can be considered additional sufferings by Christ Himself. Because Christ’s enemies had not filled up all their hatred on Him, they turned their hatred on those who, like Paul, preached the gospel. Paul in prison as he penned Colossians, writes that the church was benefiting from the fact that he has suffered and continued to suffer for the cause of Christ; as his sufferings furthered the spread of the Gospel. Such suffering brought him joy because the sufferings Christians endure are a continuation of what Christ endured on earth and in that sense the Christian is identified with His afflictions.
The “afflictions of Christ” are the afflictions “which Christ endured.” T.K. Abbott comments, “Paul’s description of his troubles as the afflictions of Christ in his flesh [means] that ‘Christ’s afflictions are regarded as the type of all those that are endured by His followers on behalf of the Church” J.B. Lightfoot has written, “It is a simple matter of fact, that the afflictions of every saint and martyr do ‘fill up’ the afflictions of Christ. The Church is built up by repeated acts of self-denial in successive individuals and successive generations.” As Christ suffered persecution, there is plenty of suffering left for Paul and for each Christian to endure.
So in summary, when Paul writes about identifying with or filling up what is lacking in the sufferings of Christ, there is at the heart of his thinking a commitment to suffer to the extent that Christ suffered, without, of course, any idea of atoning value, for the purpose of the spread of the Gospel. He is saying that the sufferings he and other saints endure in the spreading of the Gospel are in the interest of the Body of Christ and knowing that one can rejoice. Until we join Christ in glory, Christians will experience some of the same suffering and persecution that Jesus Christ experienced. Paul’s motivation for enduring and rejoicing in the midst of persecution was that it benefited and built up the Body of Christ.
The French philosopher and theologian Blaise Pascal once remarked that “Jesus will be in agony until the end of the world.”  Pascal reminds us that as we follow Jesus the enemies of the Savior, when they persecute Christ’s followers, they are persecuting Him. As previously stated, whatever the Church suffers can be considered additional sufferings by Christ Himself. The Church is the Body of Christ and when His Body is suffering, the Savior is in agony, as well. Yet the afflictions Christians endure for the cause of Christ, in the end is a demonstration of the triumph of divine love for those whose salvation Jesus purchased with His precious blood.
 T.K. Abbott, The Epistles of Paul to the Ephesians and to the Colossians (Edinburgh: T & T Clark), 232.
 J.B. Lightfoot, Colossians (London, 1879), 166.
 Blaise Pascal, The Provincial Letters (New York: Hurd and Houghton, 1866), 48.