When he was on earth, Jesus sacrificed himself for others because he loved them. This brought him terrible suffering. But his suffering led to glory when God raised him from the dead.
Paul followed Jesus’ example of serving others out of love. He served them by sharing the truth about Jesus. Paul shared all of God’s word. Paul preached to Jews and Gentiles that Jesus is Lord of everything.
He was put in prison for preaching this. He was suffering because he had followed Jesus’ example faithfully. In this way, his suffering was part of Jesus’ suffering. Suffering didn’t stop Paul from working hard as an apostle. Jesus gave him strength to keep working.
Even from prison he worked hard to help believers grow strong in the faith. He didn’t want them to be fooled by teachings that sounded good but weren’t true. He wanted believers to fully understand that Jesus Christ lives inside of them. This truth is what Paul called the mystery of Christ.