Paul tells the Philippians in Philippians 3 to rejoice in the Lord, not in how good they think they are (Philippians 3:1). Paul uses himself as an example to show that he had many reasons to have faith when he was in his flesh (Philippians 3:4–6). Still, he didn't put any faith in what he had done (Philippians 3:3). Instead, he looked at how powerful the resurrection of Jesus was (Philippians 3:10). He thought that everything that most people would call a gain was a loss (Philippians 3:7). Everything else was trash compared to Christ (Philippians 3:8). Paul only cared about Christ. Paul put his trust and joy in the Lord and looked to the power of Jesus' resurrection: "I want to know Christ—yes, I want to know the power of his resurrection."
Paul didn't put his trust in his own flesh. Instead, he put his trust in what God had done in his life to make him righteous, not through the Law of Moses but through faith in Jesus Christ (Philippians 3:9). Paul knew that the only way to be truly righteous was to believe in Jesus (Romans 3:20; Ephesians 2:8–9). Paul didn't think much of what he had done for himself. He thought it was the most important thing that God had given him Christ (Philippians 3:8) and made him righteous (Philippians 3:9). Paul would be found in Christ (Philippians 3:9), and he knew Christ and the power of His resurrection (Philippians 3:10).
Paul even shared in Jesus' sufferings because he was made to look like Christ's death (Philippians 3:10). This is another way to say that Paul and everyone else who believed in Christ were counted as having died with Christ. Paul looked forward to being raised from the dead because of these things (Philippians 3:11). Paul said in another place that if we died with Christ, we believe we will also rise with Him and be glorified with Him (Romans 6:6–8). This was what made His resurrection so powerful.
Jesus' death took the place of sin. All people who believe in Him are saved from death and from being separated from God. One day, Jesus will be shown in all his glory, and we will be shown with him (Colossians 3:4). Paul knew this to be true. Everything else in life can't compare to how great it is to know Christ. Christ's resurrection showed that He has the power to beat death and give all who believe in Him real life in the end. Paul valued this knowledge more than anything else in his life.
Paul had faith in Jesus and could be happy in the Lord because of the power of the resurrection. Paul could tell all Christians to rejoice in the Lord (Philippians 3:1) and to put knowing Christ above everything else (Philippians 3:8), considering everything else to be worthless in comparison.