3:1-7 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.

4. understand my knowledge—"perceive my understanding" [Alford], or "intelligence." "When ye read," implies that, deep as are the mysteries of this Epistle, the way for all to understand them is to read it (2Ti 3:15, 16). By perceiving his understanding of the mysteries, they, too, will be enabled to understand.

the mystery of Christ—The "mystery" is Christ Himself, once hidden, but now revealed (Col 1:27).