Ephesians 3:1

"For this reason" refers to what Paul had said about God’s blessings that are now the possession of both Gentile and Jewish believers. Since God has blessed us so greatly, Paul prayed that his readers would comprehend fully the extent of God’s love for them (Ephesians 3:14-21).

His reference to himself as Christ’s prisoner for the sake of the Gentiles led him to digress and explain why he was such. When Paul wrote this epistle, he was under house arrest in Rome. This imprisonment had resulted from his service for Christ, specifically his ministry among Gentiles, for which the Jews had mobbed him in Jerusalem (cf. Acts 21:21; Acts 21:18; 2 Timothy 1:11-12). The apostle regarded his imprisonment as God’s will for him then.