In Eph. 3:1-3 “the dispensation of the grace of God” is specifically called "the mystery" (i.e., secret). It is thus designated "the mystery" because 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).

"The mystery" has two aspects - corporate and personal or individual.

• THE CORPORATE BODY OF CHRIST: In Eph 2:11-16 we see the mystery is the joint body of Jew & Gentile together equally in "one body" making the "one new man."

Ephesians 2:11-16 Wherefore remember, that ye being in time past Gentiles in the flesh, who are called Uncircumcision by that which is called the Circumcision in the flesh made by hands; ¹² That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world: ¹³ But now in Christ Jesus ye (Gentiles) who sometimes were far off are made nigh by the blood of Christ. ¹⁴ For he is our peace, who hath made both one (Jew & Gentile), and hath broken down the middle wall of partition between us; ¹⁵ Having abolished in his flesh the enmity, even the law of commandments contained in ordinances; for to make in himself of twain one new man, so making peace; ¹⁶ And that he might reconcile both unto God in one body by the cross, having slain the enmity thereby:

Believing Jews and believing Gentiles stand on level ground as equals "in Christ. They are essentially as all Gentiles - being "none righteous, no not one" in themselves.

Galatians 3:28  There is neither Jew nor Greek (Gentile), there is neither bond nor free, there is neither male nor female: for ye are all one in Christ Jesus.

• THE INDIVIDUAL MEMBERS OF THE BODY OF CHRIST is seen ever so clearly in Col 1:26-27 when we consider that we are the body's "members in particular."

Colossians 1:26-27  Even the mystery which hath been hid from ages and from generations, but now is made manifest to his saints: ²⁷ To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:

1 Corinthians 12:27 (KJV) Now ye are the body of Christ, and members in particular.

1 Corinthians 12:27 (AMP) Now you [collectively] are Christ's body and [individually] you are members of it, each part severally and distinct [each with his own place and function].

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” to preach (Col 1:27b-28a, 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).

Ephesians 1:9 Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself: