Does not Eph. 3:1-6 which you quoted earlier explain exactly what the mystery given to Paul was?
There were several mysteries, not just one (1 Corinthians 4:1 KJV)
Eph 3:1 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles,
Eph 3:2 If ye have heard of the dispensation of the grace of God which is given me to you-ward:
Eph 3:3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words,
Eph 3:4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ)
Eph 3:5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
Eph 3:6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Was not the mystery that the Gentiles would be fellowheirs of the same body? And not that of a different gospel, which Paul warned about someone preaching?
I think so...
The mystery of Ephesians 3 is the mystery of the gospel Ephesians 3:6 KJV
Specifically, these "you Gentiles" are fellowheirs and of the same body and partakers of God's promise in Christ by the gospel with the "we" "who first trusted in Christ"
Ephesians 1:12 That we should be to the praise of his glory, who first trusted in Christ.
beginning with the the apostle Paul
1 Timothy 1:16 Howbeit for this cause I obtained mercy, that in me first Jesus Christ might shew forth all longsuffering, for a pattern to them which should hereafter believe on him to life everlasting.
and including the Romans, Corinthians, Galatians and Thessalonians
These were those to whom Paul was first sent. Here is Paul recalling what the risen ascended Lord Jesus Christ told him in Acts 9. Look at what it says at the end of verse 17 taking note of "unto whom NOW I send thee.
Acts 26:16-17 But rise, and stand upon thy feet: for I have appeared unto thee for this purpose, to make thee a minister and a witness both of these things which thou hast seen, and of those things in the which I will appear unto thee; 17 Delivering thee from the people, and from the Gentiles, unto whom now I send thee,
Here they are:
Acts 13:26 Men and brethren, children of the stock of Abraham, and whosoever among you feareth God, to you is the word of this salvation sent.
Here they are again~"to the Jew first and also to the Greek":
Romans 1:16 For I am not ashamed of the gospel of Christ: for it is the power of God unto salvation to every one that believeth; to the Jew first, and also to the Greek.
But unto them which are called, "both Jews and Greeks" (this is the first group)
1 Corinthians 1:24 But unto them which are called, both Jews and Greeks, Christ the power of God, and the wisdom of God.
They had a hope. They were in the commonwealth of Israel. They were in the promise.
Galatians 3:29 And if ye be Christ's, then are ye Abraham's seed, and heirs according to the promise.
All Greeks were Gentiles, but not all Gentiles were Greeks.
The Ephesians were not "Abraham's seed, and heirs according to the promise". These Gentiles were OUTSIDE of the commonwealth of Israel and strangers from the covenants of promise (such as we). These Gentiles in time past had no hope and were without God in the world!
Ephesians 2:11-12 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:
These Ephesians , the Gentiles to whom Paul wrote the letter are of the "ye" whom "also trusted"
Ephesians 1:13-14 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise, 14 Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.
These are Gentiles that Paul became a prisoner for when he revealed the vision:
Please take note of "for I will send thee" far hence unto the Gentiles.
Acts 22:21-22 And he said unto me, Depart: for I will send thee far hence unto the Gentiles. 22 And they gave him audience unto this word, and then lifted up their voices, and said, Away with such a fellow from the earth: for it is not fit that he should live.
Ephesians 3:1-6 For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, 2 If ye have heard of the dispensation of the grace of God which is given me to you-ward: 3 How that by revelation he made known unto me the mystery; (as I wrote afore in few words, 4 Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 5 Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; 6 That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel:
Together (the twain) make up the one new man
"But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ... For he is our peace, who hath made both one... for to make in himself of twain one new man, so making peace;"
Ephesians 2:13-15 But now in Christ Jesus ye who sometimes were far off are made nigh by the blood of Christ. 14 For he is our peace, who hath made both one, 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;
