The Mystery of Christ - Ephesians 3:5

Ephesians 3:1-6 For this reason I, Paul, a prisoner for Christ Jesus on behalf of you Gentiles - assuming that you have heard of the stewardship of God's grace that was given to me for you, how the mystery was made known to me by revelation, as I have written briefly. When you read this, you can perceive my insight into the mystery of Christ, which was not made known to the sons of men in other generations as it has now been revealed to his holy apostles and prophets by the Spirit. This mystery is that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.

The mystery spoken of here is the fact that the wild branches (Gentiles) would be grafted into the olive tree (Rom. 11:11-24). The Gentiles would be fellow heirs (Eph. 3:6). The Gentiles would share the same promises as the Jews. It speaks of the union of Jews and Gentiles in the church (Eph. 2:14-18).

The prophets referred to here are New Testament prophets (Rom 12:6; 1 Cor. 12:28; Eph. 2:20; 4:11; cf. Acts 11:27; 13:1-2; 15:32; 19:6; 21:9-10; Rev 1:3; 10:11; 16:6; 18:20, 24; 19:10; 22:6-10, 18-19).

The Spirit of the Reformation Study Bible states that God's saving purpose for Gentiles was for the most part, [but not completely], hidden from them prior to the coming of Christ. Previous generations were allowed to "walk in their own way" (Acts 14:16; cf. Rom. 1:24-32; Eph. 2:12). The Old Testament did speak in shadows, signs, and hints of the day when God would bring the Gentiles into the kingdom of God through the Messiah (Gen. 12:3; Zech. 9:9-10). Yet, Paul himself had to learn that the majority of the Jews would reject the gospel and that the Gentiles would believe in large numbers (Acts 13:46). In this sense, the matter and degree to which the inclusion of the Gentiles would take place in the New Testament period had been largely hidden. It was revealed only to, and explained by, the apostles and prophets of the New Testament (Eph. 3:5-6).

So, the full extent of the ingraftng of Gentiles was not fully understood or realized. However, it was not completely unknown in the Old or New Testaments (Gen. 17:4-7; Luke 2:31-32). It was anticipated by the prophets. Isaiah states, "The Lord Almighty will bless them, saying, 'Blessed be Egypt my people, Assyria my handiwork, and Israel my inheritance'" (Isa. 19:25, cf. 42:5-9). There are glimpses of its reality in the Old Testament (Ruth 1:16; 2 Sam. 15:18-21; also Nineveh in Jonah) and in the New Testament before the death and resurrection of Christ (Matt. 15:21-28; feeding of 4000 in a Gentile region of Decapolis - Mark 7:31; 8:1 ff). In the New Testament, even Paul points back to the Old and says that the Abrahamic covenant included all who were of a like faith with Abraham, including Gentiles (Rom. 4:17-18; cf. Gen. 17:4-7). As Paul told Agrippa in Acts 26:22-23:

But I have had God's help to this very day, and so I stand here and testify to small and great alike. I am saying nothing beyond what the prophets and Moses said would happen - that the Christ would suffer and, as the first to rise from the dead, would proclaim light to his own people and to the Gentiles."

Related Answer: The Old/New Testament Church