Albert Barnes Commentary Ephesians 3:4

"whereby, when ye read, ye can perceive my understanding in the mystery of Christ;" — Ephesians 3:4 (ASV)

Whereby when ye read. By simply reading this, you may understand the view I hold concerning the plan of salvation, and the knowledge I have of God's method of saving people, particularly His intention regarding the salvation of the Gentiles.

In the mystery of Christ. This does not refer to anything mysterious in the person of Christ, or the union of the Divine and human nature in Him, or to anything difficult to understand in the work of the atonement. It means the previously concealed doctrine that through the Messiah the Gentiles were to be received into the same privileges as the Jews, and that the plan of salvation was to be made equally free for all. This great truth had been previously concealed, or only partially understood, and Paul says that he was appointed to make it known to the world. His knowledge on the subject, he says, could be understood by what he had written, and from that they could judge whether he was qualified to state and defend the doctrines of the Gospel.

Paul evidently supposed that the knowledge he had on this subject was of eminent value, that it was possessed by few, and that it was important to understand it. Therefore, he dwells on it. He speaks of the glory of that truth. He traces it back to the counsels of God. He shows that it entered into His eternal plans, and he evidently felt that the truth he had communicated in the earlier part of this epistle was among the most important that could come before the mind.