When did Paul pen Romans? I think it was within 3 years of Ephesians. I guess it depends when we see the start of the Church. Is it Acts 28? Or Acts 13 where Paul declares for the first time that he is going to the Gentiles like he did in Acts 28. I have to ask for the timeline because no one has put a stake in the ground where the Gospel or dispensation changed. Was the mystery revealed during the first prison years in Rome? I think this point is important because we would have to assume Paul established churches under one dispensation or gospel and others under another. Or did he plant these churches and then rewrite epistles superseding the previous?
Now I make known to you, brethren, the gospel which I preached to you, which also you received, in which also you stand, by which also you are saved, if you hold fast the word which I preached to you, unless you believed in vain. For I delivered to you as of first importance what I also received, that Christ died for our sins according to the Scriptures,and that He was buried, and that He was raised on the third day according to the Scriptures, (1 Corinthians 15.1-4)
Do you see the difference in Paul’s gospel? Where is repentance? Where is baptism? They are not there. What is present is an emphasis on belief. Paul’s gospel was that Christ died for your sins and rose from the the dead. Will you believe it? If so, you are saved. This is God’s promise.
Peter’s gospel to the Jews was that they had murdered their Messiah and needed to repent and be baptized (Acts 2.36-38, 41). Did Peter or the Twelve preach Christ crucified for salvation? No. The cross for Peter was something to be repented of, not a message of hope and salvation (Acts 2.22-24; 2.36-39; 3.13-20).
Not until the Jerusalem Council (c. 49-51 A.D.) did this begin to change. After much discussion and dissent, Peter, who apparently had remained quiet during the debate (what a character change!) made a remarkable statement:
After there had been much debate, Peter stood up and said to them, “Brethren, you know that in the early days God made a choice among you, that by my mouth the Gentiles would hear the word of the gospel and believe.” And God, who knows the heart, testified to them giving them the Holy Spirit, just as He also did to us; and He made no distinction between us and them, cleansing their hearts by faith. ”Now therefore why do you put God to the test by placing upon the neck of the disciples a yoke which neither our fathers nor we have been able to bear? ”But we believe that we are saved through the grace of the Lord Jesus, in the same way as they also are.” All the people kept silent, and they were listening to Barnabas and Paul as they were relating what signs and wonders God had done through them among the Gentiles. (Acts 15.8-12)
At the critical moment, God the Holy Spirit moved Peter to recall the time he went to the house of Cornelius, the Roman centurion, a Gentile, many years before (Acts 10.1-48). Peter, who had been listening to the arguments, rose to Paul’s defense in the fierce debate over whether Gentiles had to be circumcised and keep the Law of Moses to be saved. Had it not been for Peter, Paul would have faced severe opposition from both believers and unbelievers. What is most remarkable is Peter’s statement: “But we believe that we are saved through the grace of the Lord Jesus, in the same way as they also are.” What a switch! Jews saved like Gentiles, not Gentiles like Jews? Amazing! What was the result? The Jews who had argued against Paul became silent and listened to Paul and Barnabas.
(All of) This stuff comes from here and many number of other sources on bible doctrine...I stumbled across this site just tonight...
I don't know that anyone knows the exact timeline but some seem to think they are close...One thing is clear to me, there was a transition between Pentecost and the full fledged Gentile church...