The Philippian believers could be full of joy because they belonged to the Lord. Yet some people taught that Gentile believers had to be circumcised to belong to Jesus.
Paul explained that people shouldn’t trust in circumcision or anything else that humans can do. Nothing that human beings do can make them right with God. Paul had done many things that made him look like an important Jew. But those things didn’t save him.
God saves people when they believe that Jesus is Lord and Christ. Paul had joy while he was alive because he knew Christ. He was already living as a citizen of heaven. His goal for the future was to be with Jesus for ever.
Jesus will return to earth from heaven. He will bring everything on earth under his control. God will raise Jesus’ followers from the dead. At the resurrection they will have new bodies like Jesus has. Paul longed for this. He wanted the Philippians to follow his example and have that goal too.