Paul claims to 'fill up what is lacking in Christ's afflictions.' What does this startling phrase really mean, and how can anything be 'lacking' in Christ's work?
This doesn't mean Christ's suffering was insufficient for salvation. Instead, Paul is speaking about the ongoing work of Christ in the world through His body, the Church. His suffering in his own body contributes to the completion of Christ's redemptive work as it extends to His people.
Completing Christ's Work in the World
Christ's atoning sacrifice is complete and perfect. However, the application of that work and the extension of His kingdom often involve the suffering of His followers. Paul's unique apostleship, suffering for the Gentiles, is part of this ongoing process of bringing Christ's redemptive work to its full fruition.
The Church as Christ's Body
The phrase "for the sake of his body, that is, the church" is crucial. Paul's suffering isn't for his own merit but for the benefit and growth of the church. It's through the collective experience of Christ's followers, enduring hardship in His name, that His work continues to impact the world.