That the Gentiles should be fellow–heirs, and of the same body and partakers of his promise in Christ by the gospel [Eph. 3:5–6].

Paul certainly makes it clear here that this was not revealed to him alone.
Now he clarifies what he means by the mystery. There is a sharp contrast between the sons of men in past generations and the apostles and prophets of the church. No one in the Old Testament had a glimmer of light relative to the church. It is now revealed to His Holy apostles. They are “holy” because they have been set aside for this office by God. The “prophets” are definitely New Testament prophets.
The “Spirit,” the Holy Spirit, is the teacher of this mystery. This is what the Lord Jesus promised when He told His disciples of the coming of the Holy Spirit. “All things that the Father hath are mine: therefore said I, that he shall take of mine, and shall shew it unto you” (John 16:15).
What precisely is the mystery? It is not the fact that Gentiles would be saved. The Old Testament clearly taught that Gentiles would be saved. Let me cite several passages: “And in that day there shall be a root of Jesse, which shall stand for an ensign of the people; to it shall the Gentiles seek: and his rest shall be glorious” (Isa. 11:10). Another: “And the Gentiles shall come to thy light, and kings to the brightness of thy rising” (Isa. 60:3). Isaiah also wrote: “I the LORD have called thee in righteousness, and will hold thine hand, and will keep thee, and give thee for a covenant of the people, for a light of the Gentiles” (Isa. 42:6). Zechariah also mentions it: “And many nations shall be joined to the LORD in that day, and shall be my people: and I will dwell in the midst of thee, and thou shalt know that the LORD of hosts hath sent me unto thee” (Zech. 2:11). And Malachi: “For from the rising of the sun even unto the going down of the same my name shall be great among the Gentiles; and in every place incense shall be offered unto my name, and a pure offering: for my name shall be great among the heathen, saith the LORD of hosts” (Mal. 1:11).
If the mystery is not that the Gentiles would be saved, what is the mystery? Mark it carefully. The mystery was that the Gentiles and Israel were placed on the same basis. By faith in Christ they were both brought into a new body which is the church. Christ is the Head of that new body.
Therefore, now there is a threefold division in the human race:
All people were Gentiles from Adam to Abraham—2000 years (plus)
All people were either Jews or Gentiles from Abraham to Christ—2000 years
The threefold division is Jews, Gentiles, and the church from the Day of Pentecost to the Rapture—2000 years (plus)
Paul referred to this threefold division when he said, “Give none offence, neither to the Jews, nor to the Gentiles, nor to the church of God” (1 Cor. 10:32). Paul included the whole human family when he said that.
The church is not in the Old Testament de facto, although there are types of it in the Old Testament. Christ said, “… upon this rock I will build my church …” (Matt. 16:18, italics mine), and when He spoke that, it was still future. The church began on the Day of Pentecost, after Christ had returned to heaven. To say that the church began beyond the Day of Pentecost makes the church a pair of Siamese twins—a Jewish church and a Gentile church coexisting. It is true that the church was all Jewish when it began, but there was a period of transition when Gentiles were brought into it. The church is one body, made up of both Jew and Gentile, and Christ is the Head of that body.  McGee, J. V. (1991). Thru the Bible commentary: The Epistles (Ephesians) (electronic ed., Vol. 47, pp. 95–97). Nashville: Thomas Nelson.

Eph 3:6 Gentiles should be fellow heirs. A summary of 2:11–22. See notes on 1 Cor. 12:12, 13; Gal. 3:29.

12:12 body … members. Paul used the human body as an analogy (cf. 10:17) for the unity of the church in Christ. From this point on to v. 27, he used “body” 18 times (cf. Rom. 12:5; Eph. 1:23; 2:16; 4:4, 12, 16; Col. 1:18).
12:13 baptized. The church, the spiritual body of Christ, is formed as believers are immersed by Christ with the Holy Spirit. Christ is the baptizer (see note on Matt. 3:11) who immerses each believer with the Spirit into unity with all other believers. Paul is not writing of water baptism. That outward sign depicts the believer’s union with Christ in His death and resurrection (see notes on Rom. 6:3–5). Similarly, all believers are also immersed into the body of Christ by means of the Holy Spirit. Paul’s point is to emphasize the unity of believers. There cannot be any believer who has not been Spirit-baptized, nor can there be more than one Spirit baptism or the whole point of unity in the body of Christ is convoluted. Believers have all been Spirit-baptized and thus are all in one body. See notes on Eph. 4:4–6. This is not an experience to seek, but a reality to acknowledge. See also notes on Acts 8:17; 10:44, 45; 11:15–17. drink into one Spirit. At salvation, all believers not only become full members of Christ’s body, the church, but the Holy Spirit is placed within each of them (Rom. 8:9; cf. 6:19; Col. 2:10; 2 Pet. 1:3, 4). There is no need (or divine provision) for any such thing as a second blessing, a triumphalistic experience of a deeper life, or a formula for instantly increased spirituality (cf. John 3:34). Christ’s salvation provision is perfect and He calls only for obedience and trust in what has already been given (Heb. 10:14).

3:29 Abraham’s seed. See note on v. 7. Not all physical children of Abraham are the “Israel of God” (cf. 6:16), that is, true spiritual children of Abraham (Rom. 9:6–8). Gentile believers who are not physical children of Abraham are, however, his spiritual children in the sense that they followed the pattern of his faith (see note on Rom. 4:11). heirs according to the promise. All believers are heirs of the spiritual blessing that accompanied the Abrahamic Covenant—justification by faith (Gen. 15:6; cf. Rom. 4:3–11).

MacArthur, J., Jr. (Ed.). (1997). The MacArthur Study Bible (electronic ed., p. 1794). Nashville, TN: Word Pub.