The mystery revealed to the apostles: the nations are fellow heirs in one body — announced in the apostolic age, not deferred to a later track of history.
Paul calls it a mystery “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” (Ephesians 3:5–6): that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus through the gospel.
The nations’ blessing was promised from Abraham onward (Genesis 12:3; 22:18). What was not plain was the form: one body in the Messiah without the wall of ordinances — inclusion by faith, not by becoming proselytes under Moses’ administration.
Acts is the narrative of the mystery becoming public: the Ethiopian, Cornelius, Antioch, the missionary journeys, the council’s letter, Paul’s defense that God opened a door of faith to the Gentiles. The Spirit’s falling on the uncircumcised forces the issue: if God gave the same gift, who can refuse recognition?
Galatians fights the counter-mystery — the attempt to drag Gentile believers under the Law as if Christ were not enough. Justification by faith and the one family of Abraham are the same argument from different sides.
Gentile inclusion is not a temporary parenthesis before God returns to a separate ethnic plan as the main story. The one new man is the story. The olive tree of Romans 11 has natural branches broken off for unbelief and wild branches grafted in by faith; arrogance is forbidden either way, but the tree is one.
Fellow heirs. Same body. Same promise.
The mystery’s content is unity in Christ now — the apostolic “now” of Ephesians 3 — not a delay of the nations’ inheritance.
When the Temple fell, the institutional machinery that had policed Gentile distance was gone. The church already knew the theology; history removed the rival center. The nations enter by the gospel; the remnant of Israel enters by the same faith; the dividing wall does not get a second life.