Ephesians 3:3-6

(3) How that by revelation he made known unto me the mystery; (as I wrote afore in few words, (4) Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) (5) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; (6) That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:

The Apostle, no doubt, when speaking of having written to them before, alludes to the first Chapter particularly in this Epistle, in which he hath treated of those mysterious doctrines, of election, predestination, the acceptation of the Church in Christ, before the foundation of the world; together with redemption by Christ, and regeneration by the Spirit, during the time-state of the Church. But, what I would yet more particularly desire the Reader to regard, in this statement of Paul, is, what he saith in relation to, the mystery of these grand things being kept secret in times past, compared to the revelation now made, unto the Apostles, and Prophets, by the Spirit. The Jews knew nothing of God's design, in relation to the Gentiles. Though, had they understood their own Scriptures, they would have discovered, that, from the beginning, the Church of God was One, and that the Gentiles should be fellow-heirs of the same body. Christ's Church could be but one. And Christ himself the One Head, and Husband of his body the Church; and Jehovah's salvation to the ends of the earth. Isaiah 49:6; Song of Solomon 6:9. But, until the ascension of Christ, and the descension of the Holy Ghost; these glorious truths, were not so openly revealed, as they, were then by the Spirit.