Heir, regarding your above to Jerry, as you know, you and I also differ somewhat in our understanding of that. The following is mine.
In Romans 1, Paul is asserting that he was separated unto the Gospel of God...concerning His Son; Who was promised afore by His prophets in the Holy Scriptures.
He is not asserting he was separated unto God's prophesied gospel of God concerning His Son, rather; unto the gospel of God concerning His Son that Paul was given to preach; "the preaching of Jesus Christ according to the revelation of the Mystery, which was kept secret since the world began, but now is made manifest..." Rom. 16:25.
Continuing on that assertion in Romans 1, he goes into his desired intent in writing Romans...
First, Paul was separated unto the gospel of God
Romans 1:1 Paul, a servant of Jesus Christ, called to be an apostle, separated unto the gospel of God,
as in
separated for the work that he was called to do, much like:
Acts 13:2 As they ministered to the Lord, and fasted,
the Holy Ghost said, Separate me Barnabas and Saul for the work whereunto I have called them.
It is a sending forth:
Acts 13:3 And when they had fasted and prayed, and laid their hands on them, they sent them away.
Acts 13:4 So
they, being sent forth by the Holy Ghost, departed unto Seleucia; and from thence they sailed to Cyprus
In the face of his having been unable to do what he had been able to do, for example, in Acts 20, at Ephesus - ground them in the mutual faith they too had been called unto, Rom. 1:7....
In absence of that, he writes Romans as his means of some spiritual gift in written form, whereby they might be established in the mutual faith both of you (they were already Mystery Body members) and of me (as was Paul).
The Romans were called to be saints. It doesn't necessarily mean they were members of the Body of Christ at that moment. These Romans had a faith that was spoken of throughout the whole world, but it was not the mutual faith of both Paul and them (Romans 1:7-8 KJV). Paul longed to see them, that he may impart unto them some spiritual gift to the end they may be established; that is, that he may be comforted together with them by the mutual faith both of them and him (Romans 1:9-12 KJV ). Them hearing and believing would make them have a mutual faith!
The "some spiritual gift" can be shown to be the gospel of Christ and the reason that Paul longed to see them was to impart it to them. I believe it's because they had never heard it. They were not yet established. The gospel of Christ is what would establish them (1 Thessalonians 3:2 KJV).
Romans 16:25-27 KJV Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: 27 To God only wise, be glory through Jesus Christ for ever. Amen.
(side note: That is not the gospel of God that you mixed together in the beginning of your post to me. The gospel of God is the WHO of Jesus Christ and that God raised Him from the dead. It is not the WHY of the cross; the gospel of Christ, the power of God unto salvation to every one that believeth, Paul's "my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, but now is made manifest,...")
Because these Roman had never heard the gospel of Christ (1 Corinthians 15:1-4 KJV):
Romans 1:15-17 KJV So, as much as in me is, I am ready to preach the gospel to you that are at Rome also. 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. 17 For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith.