In Eph. 3:1-3 “the dispensation of the grace of God” is specifically called “the mystery” (i.e., secret). It is thus designated for two reasons:
1⃣ It had been “kept secret since the world began, but now,” through Paul, had been “made manifest” (Rom. 16:25). “In other ages” it was “not made known” (Eph. 3:5). Rather, “from the beginning of the world” it had been “hid in God” (Ver. 9), “hid from ages and from generations, but now… made manifest to His saints” (Col. 1:26).
2⃣ It was at the same time the explanation, the key, to all God’s good news, including that which had been proclaimed in ages past. It explained how it was that Abel could be declared righteous by bringing an animal sacrifice, “God testifying of his gifts” (Heb. 11:4), how Noah could become “an heir of… righteousness” by building an ark (Heb. 11:7), how anyone could be saved under the dispensation of the Law, and how it is that we can be saved today by grace through faith alone.
Thus we have in Paul’s epistles, not only the gospel [good news] of “the secret” (Eph. 3:1-3), but at the same time, “the secret of the gospel” (Eph. 6:19,20).
This great secret, revealed to and through Paul, has rightly been called the capstone of divine revelation, for it concerns God’s eternal purpose in Christ. Through Paul, the chief of sinners saved by grace, God has now made this glorious secret known to us (Eph. 1:9) that we, in turn, might make it known to others (Eph. 3:9). source