If indeed you have heard of the dispensation of the grace of God which was given me for you, (3) how by that revelation He made known to me the mystery (as I have briefly written already),….(5) which in other ages was not made known to the sons of men, as it has now been revealed by the Spirit to His holy apostles and prophets: (6) that the Gentiles should be fellow heirs, of the same body, and partakers of His promise in Christ through the gospel,

The letter to the Ephesians was written by the apostle Paul. Paul received the office of apostleship by direct revelation from Jesus Christ (not by hearsay) concerning the purpose of redemption. It was not attainable by human reason. The special mystery was the union of Jews and Gentiles as joint partakers of the promise of redemption. This union was frequently predicted by the ancient prophets but was not revealed so fully or clearly as under the gospel.

The mystery of the new dispensation that Gentiles are fellow heirs with the Jews, formerly thought the calling of Gentiles in the Old Testament led to the general impression that they were to partake by becoming Jews merged into the old theocracy which was to remain in all its peculiarities. But in the Day of Pentecost it was revealed that the old theocracy itself was to be abolished and a new form introduced designed and adapted for all mankind. There was no more distinction between Jews and Gentiles who now have the same rights to inheritance – all benefits of the covenant of grace, knowledge of truth, justification, adoption, sanctification; indwelling of the Spirit and life everlasting.

Christ Himself is the great mystery of godliness – God manifested in the flesh. He is the revelation of the secret purpose of God which had been hid. The whole plan of salvation designated the mystery of Christ includes far more than the Gentiles were fellow citizens of saints. The only essential and indispensable condition of participation in the benefits of redemption is union with Christ. This union is brought about by the gospel ; not by birth, outward rite, union with an external body but the gospel received and appropriated by faith that we are united to Christ and thus made heirs of God. Wow.