Charles Hodge on Ephesians 3:4

V. 4. By reading what he had written, they could judge of his knowledge of the mystery of Christ. πρὸς ὃ, according to which. What he had written might be taken as the standard or evidence of his knowledge. Mystery of Christ, may mean the mystery or revelation concerning Christ; or of which he is the author (i. e. of the secret purpose of redemption), or which is Christ. Christ himself is the great mystery of godliness, God manifest in the flesh. He is the revelation of the μυστήριων or secret purpose of God, which had been hid for ages. Thus the apostle in writing to the Colossians says: "God would make known the riches of the glory of the mystery among the Gentiles; which (i. e. the mystery) is Christ in you, the hope of glory." Col. 1, 27.

What Paul had written respecting the calling of the Gentiles in the preceding chapter, was an indication of his knowledge of the whole plan of salvation—here designated as "the mystery of Christ," which includes far more than the truth that the Gentiles were fellow-citizens of the saints. It has the same extensive meaning in Col. 4, 3, where Paul prays that God would open a door of utterance for him "to speak the mystery of Christ." This verse is, therefore, virtually a parenthesis, in so far as the relative ὅ at the beginning of the next verse refers to the word μυστήριων in v. 3; or if referred to that word as used in v. 4, it is to it as including the more limited idea expressed in v. 3.