Romans 1:2 — (Which He had promised afore by His prophets in the holy Scriptures.)
My thoughts are in process here, so I might change my mind at some point. But as I was studying this verse I decided something. Paul sometimes uses the word “gospel” to refer to a more general good news that God promised in the Old Testament and then delivered through Christ. At other times, Paul uses the word “gospel” to refer more specifically to those aspects that are part of the mystery that was given to him and to nobody else.
Which is Paul referring to in Romans 1:2? I think it’s the more general sense — good news promised but not detailed. In 1 Corinthians 2:9, Paul quotes Isaiah 64:6: But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love Him. Isaiah was one of God’s prophets, the book he wrote is part of the holy Scriptures (a term that is almost always referring to the Old Testament). Isaiah prophesied wonderful things, good news, but he didn’t get into specifics because he didn’t know the specifics.
In John 5:46, Jesus Himself talked to the Jews about the Old Testament prophecies that were about Him: For had ye believed Moses, ye would have believed Me: for he wrote of Me.
The Jews (as a nation) rejected Christ. They didn’t see Him as the fulfillment of the O.T. prophecies.
So a new, more specific, more detailed message was given to Paul. Right after the verse in which he quotes Isaiah, Paul wrote: But God hath revealed them unto us by His Spirit: for the Spirit searcheth all things, yea, the deep things of God (1 Corinthians 9:10).
God’s good news concerning His Son, the Redeemer, was promised in the Old Testament, but it wasn’t fully revealed until it was given to Paul.