What is the mystery?
The mystery refers to the purpose of God in Christ, which is addressed by the Apostle Paul in the book of Ephesians:
“9 having made known to us the mystery of His will, according to His good pleasure which He purposed in Himself, 10 that in the dispensation of the fullness of the times He might gather together in one all things in Christ, both which are in heaven and which are on earth—in Him.“
Ephesians 3:6 AMP
“6 [it is this:] that the Gentiles are now joint heirs [with the Jews] and members of the same body, and joint partakers [sharing] in the [same divine] promise in Christ Jesus through [their faith in] the good news [of salvation].”
The Apostle Paul’s mission was to enlighten all people about the mystery of God’s Grace, accessed through faith in Christ – Ephesians 3. The mystery is associated with God’s purpose to bring all things together in Christ [people including Jews and Gentiles (none Jews) as one], both things in heaven and in earth, in Him.
What is the fellowship of the mystery?
v.9-10 “9 And to make all men see what is the fellowship of the mystery, which from the beginning of the world has been hid in God, who created all things by Jesus Christ: 10 To the intent that now the manifold wisdom of God might be made known by the church to the principalities and powers in the heavenly places, according to the eternal purpose which He accomplished in Christ Jesus our Lord.“
The fellowship of the mystery is a believer’s partaking of the promises of God in Christ [by faith], and declaring His triumph [by the manifold wisdom of God] to “the principalities and powers in the heavenly places“.
The Triumph of Christ – Ephesians 1:19-23 ; 6:10-13
“Ch.6 v. 12 For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.”
“15 And having spoiled principalities and powers, He made a show of them openly, triumphing over them in it.”
The Fellowship of the Mystery is an association with the triumph of Jesus Christ over all principality and power [things with which we wrestle], and over all things; as He is the Head of all, to the Church!