“Now I rejoice in my sufferings for you, and I am completing in my flesh what is lacking in Christ’s afflictions for His body, that is, the church.” – Colossians 1:24

The text above does not imply that Paul was trying to complete a deficiency in Christ’s redemptive work. On the cross, once and for all, Christ paid the price for our redemption. Rather, Paul was declaring a truth he learned in his first encounter with Jesus: to persecute the followers of Christ was to persecute Christ Himself. 

When someone suffers for the sake of the gospel, Christ identifies with that person in their suffering. The early Christians counted it a joy to be persecuted for their faith, for in a small way, they were participating in the suffering of Christ. When one believes in the gospel, they may face various kinds of sufferings. These sufferings are small reflections of the greater suffering of Christ, which is being manifested to the world.