What is the 'mystery' Paul reveals in Ephesians?
Paul defines the mystery in Ephesians 3:6 as Gentiles being fellow heirs, members of the same body, and co-partakers of the promise in Christ Jesus — not as the church being a secret entity hidden from the Old Testament.
When Paul talks about "the mystery" in Ephesians, he is not inventing new vocabulary — he is picking up a word with deep roots in Daniel. The Greek word mystērion (μυστήριον, G3466) appears 58 times across the canon: 27 in the New Testament, and 17 in the Septuagint's translation of Daniel. That distribution tells you something. Both Daniel and Paul are working with the same concept: a divine secret that was hidden in God's purpose and has now been revealed through a mediator.
In Daniel 2, the Aramaic word is raz (רָז, H7328) — a divine secret. It appears eight times in that one chapter, and every time, the Greek translators rendered it as mystērion. The pattern Daniel establishes is three stages: the secret exists hidden (2:27), God reveals it by his own initiative (2:28), and it is made known through a chosen mediator. Paul's language in Ephesians 3 follows exactly the same three stages. The mystery was "hidden in God" (Eph 3:9). It was made known "by revelation" (Eph 3:3). And it was revealed to Paul specifically as its mediator (Eph 3:3). The structure is the same; the vocabulary connects through the Greek translation bridge.
So what is the mystery? Paul defines it precisely:
"that the Gentiles are fellow heirs, members of the same body, and partakers of the promise in Christ Jesus" — Ephesians 3:6
Three adjectives, and every one of them carries the Greek prefix syn- (together with): synklēronomoi (G4789, "fellow heirs"), syssōma (G4954, "of the same body" — a word Paul appears to have coined), and symmetocha (G4830, "co-partakers"). You cannot be a "fellow heir" unless someone already has the inheritance. The mystery is not that the church exists as a separate entity hidden from the Old Testament — it is the specific mode of Gentile inclusion: as joint participants in what Israel already had.
Ephesians 3:5 says this was not made known in other generations "as" (hōs, ὡς) it has now been revealed. That word "as" modifies degree, not presence versus absence. The Old Testament announced the destination — Gentile blessing (Genesis 12:3), a light to the nations (Isaiah 49:6), restoration of David's tent so that all nations might seek the Lord (Amos 9:11–12). Paul reveals the mechanism: Jew and Gentile as fellow heirs, in one body, through Christ.
For the full lexical analysis of mystērion across Paul's letters and its Daniel roots, see "Rightly Dividing" — What Dispensationalism Claims and What the Text Says, section "The Mystery: What Paul Actually Reveals."
Did the Old Testament predict Gentile inclusion in God's people?
Yes, explicitly. At least eight Old Testament passages anticipate Gentile inclusion, and the New Testament authors quote or allude to six of them as fulfilled — not as a new development but as the realization of what the prophets announced.
Does Peter say Joel's prophecy was fulfilled at Pentecost?
Yes. Peter uses the equative copula 'touto estin' (this IS that) in Acts 2:16 — direct identity, not analogy — and deliberately substitutes 'in the last days' for Joel's 'after these things,' declaring the last days had arrived.
Does 'rightly dividing' in 2 Timothy 2:15 support dispensationalism?
No. The Greek word orthotomeo means 'to cut a straight road,' not 'to partition Scripture into dispensational eras.'