For this reason I, Paul, the prisoner of Christ Jesus for you Gentiles – (2) if indeed you have heard of the dispensation of the grace of God which was given to me for you, (3) how by that revelation He made known to me the mystery (as I have briefly written already, (4) by which, when you read, you may understand my knowledge in the mystery of Christ), (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. The mystery of which he is speaking is, in Christ Jesus, there is now union – equality – of Gentiles and Jews as fellow heirs of God’s promises. The revelation of this mystery was frequently predicted by prophets of the Old Testament but the form was obscure and led to the general impression that the Gentiles were to partake of the blessings of the Messiah by becoming Jews – proselytes of the old theocracy which was thought by the Jews would remain in place. But in Christ the old order was abolished and the new order was adapted equally for all men. The distinction between Jews and Gentiles was done away with.
From Abraham to Christ, God’s blessing were mostly restricted to the Jews. Gentiles were excluded from the nation of Israel and were strangers to God’s covenants of the promise; the Gentiles were alienated from God in this world. The Gentiles are now partakers of God’s plan of redemption – the only essential condition of that promise is union with Christ. With Jesus a change of the ages took place – this has happened and now the Jews and Gentiles need to realize this experience.
The truth of the gospel is the centrality of Jesus in all salvation history. This cannot be arrived at by human logic, intuition or study. God must open our eyes to the understanding of the gospel.