Ephesians 3:1

For this cause - On account of preaching this doctrine; that is, the doctrine that the gospel was to be proclaimed to the Gentiles.

I Paul, the prisoner of Jesus Christ - A prisoner in the service of the Lord Jesus; or made a prisoner in his cause. Not a prisoner for crime or debt, or as a captive in war, but a captive in the service of the Redeemer. This proves that at the time of writing this, Paul was in bonds, and there can he no question that he was in Rome. This would be more correctly rendered, “For this cause I, Paul, am the prisoner,” etc. So Tyndale renders it, “For this cause I, Paul, the servant of Jesus, am in bonds.” So also Locke, Rosenmuller, Doddridge, Whitby, Koppe, and others understand it. By this construction the abruptness now manifest in our common version is avoided.

For you Gentiles - Made a prisoner at Rome on your behalf, because I maintained that the gospel was to be preached to the Gentiles; see Acts 22:21. He was taken first to Cesarea, and then to Rome. The cause of his imprisonment and of all his difficulties was, that he maintained that the gospel was to be preached to the Gentiles; that when the Jews rejected it God rejected them; and that he was specially called to carry the message of salvation to the pagan world.