The Book of Ephesians is in the Bible's New Testament. Paul wrote this letter to the church at Ephesus, which he had led for over two years (Acts 19:10).
When Paul wrote the Book of Ephesians, Paul was in prison. But the letter is not a sad letter. Paul was excited about God's good gifts (Ephesians 1:3). Paul was pleased that the Christians in Ephesus continued to serve God. Paul wanted to encourage them. And he told them about his prayers for them (Ephesians 1:15-22, Ephesians 3:14-21).
Paul reminded the Christians in Ephesus that previously they were enemies of God. But God sent Jesus to die for them. Because of Jesus' death, they became friends of God (Ephesians 2:11-22). So, they should live lives that please God (Ephesians 4:17-32, Ephesians 5). And, they should trust God more. God had appointed teachers to help the Christians. And the Christians would learn to be more like Jesus. Troubles would come, so they needed God's protection (Ephesians 6:10-18).