6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
English Standard Version
6 This mystery isThe words This mystery is are inferred from verse 4 that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
6 The mystery is that people who have never heard of God and those who have heard of him all their lives (what I've been calling outsiders and insiders) stand on the same ground before God. They get the same offer, same help, same promises in Christ Jesus. The Message is accessible and welcoming to everyone, across the board.
New King James Version
6 that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel,
New Living Translation
6 And this is God's plan: Both Gentiles and Jews who believe the Good News share equally in the riches inherited by God's children. Both are part of the same body, and both enjoy the promise of blessings because they belong to Christ Jesus.
For having preached the doctrine of truth, the apostle was a prisoner, but a prisoner of Jesus Christ; the object of special protection and care, while thus suffering for him. All the gracious offers of the gospel, and the joyful tidings it contains, come from the rich grace of God; it is the great means by which the Spirit works grace in the souls of men. The mystery, is that secret, hidden purpose of salvation through Christ. This was not so fully and clearly shown in the ages before Christ, as unto the prophets of the New Testament. This was the great truth made known to the apostle, that God would call the Gentiles to salvation by faith in Christ. An effectual working of Divine power attends the gifts of Divine grace. As God appointed Paul to the office, so he qualified him for it.