Paul proclaims that the reason he is a prisoner in Rome is because he preached that the gospel is for both the Jews and Gentiles. In Christ, they are equal; they are one body. This revelation was given by the Holy Spirit to the apostles and had been previously hidden.
Paul realized God's grace in using him for this great and special purpose. Especially since Paul had been a top Pharisee, if he says that Gentiles are included, it's even more powerful. Paul revealed the mystery that had been hidden and was now revealed: that the Gentiles would also receive the boundless spiritual riches.
God's plan was that through the church, this manifold wisdom was revealed to the spiritual authorities. Imagine the angels gaze in wonder that in Christ and through him, we approach the throne of God with freedom and in confidence. Thus, the believers should not be discouraged by Paul's chains, but praise God for enabling Paul to preach the gospel to the various Gentile people in Rome.
Application: Father, thank you for revealing your marvelous plan of salvation. Help us to preach God's glorious salvation wherever we are.
One Word: God's manifold wisdom to save people from all nations to be one body in his kingdom