You long have held to the error that when "Christian" is mentioned you think of Gentiles but there is no record of non-Hebrew Gentiles being born-again and being Gentile.
At Pentecost three thousand Jews attending the Jewish Feast of Harves lived in Jerusalem and throughout Israel as well as visitors from all over the Roman Empire. All one need do is understand the languages that the eleven disciples were speaking:
7 And they were all amazed and marvelled, saying one to another, Behold, are not all these which speak
Galilaeans?
8 And how hear we every man in our own tongue, wherein we were born?
9
Parthians, and
Medes, and
Elamites, and the
dwellers in Mesopotamia, and in
Judaea, and
Cappadocia, in
Pontus, and
Asia,
10
Phrygia, and
Pamphylia, in
Egypt, and in the parts of
Libya about
Cyrene, and
strangers of Rome,
Jews and
proselytes,
11
Cretes and
Arabians, we do hear them speak in our tongues the wonderful works of God. Acts 2:6–11.
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These Jews were speaking at least seventeen dialects and languages. And when these Jews returned home on Roman roads they took with them a summary of Peter's sermon, the Holy Spirit of Promise (PROMISED TO ISRAEL - Joel), and their experiences and testimony of Jesus of Nazareth, Israel's claimed King and Messiah and the Prophet like unto Moses. Only Jews were at this celebration. Uncircumcised, unclean, Gentiles were not invited nor were they present. And these three thousand newly born-again Jews became Christian or Christ-followers. All Jews that looked forward to the arrival of their Messiah were Christian (Messiah.) Acts 2:47 says Jesus added to His Church hundreds and thousands daily such as should be saved. With the advent of the Holy Spirit of Promise Israel entered the era of the New Covenant prophesied by Jeremiah. And it took them time to reason out the Scripture and come to understand more and more each day about the New Covenant era Israel found themselves in.
The subject matter where "we are under grace and not the Law" has SIN as the subject Saul was writing about.
Sin.
14 For sin shall not have dominion over you: for ye are not under the law, but under grace. Romans 6:14.
Yes, there may have been religious leaders and Jews in general that may have held that their justification came from the Law until Saul - maybe Apollos and others - rightly understood that as far as their being born-again it is in this event of their new birth that was their justification and God declared them "Not Guilty!" because of what Jesus accomplished on the cross. And just because the only writings we have are Saul's Roman letter (AD 64) doesn't mean that from Pentecost (AD 32) going forward that some might have under stood in their enlightening of the Spirit things that Saul would later write about. Priests were being born-again and these men knew the Scripture. Jesus' promise that "when He is come He shall guide you into all truth" (Jn. 16:13) applied to them also. The building of the Church did not wait until Saul came on the scene with his journey's and letter-writing, especially when he penned:
16 All scripture is given by inspiration of God, and is profitable for
doctrine, for
reproof, for
correction,
for instruction in righteousness:
17 That the man of God may be perfect, throughly furnished unto all good works.
2 Tim. 3:16–17.
I'm sure there were many that understood that obeying the Law upon one's own obedience did not bring justification, but it was after they were born-again and God declared them "Not Guilty!" they looked to the Law for "instruction in righteousness" (right living.) Remember, after the road to Damascus Saul disappeared for about 14-17 years to study as a rabbi and Pharisee would study. That how he received his "revelation" (Gal. 1.) When we study under the anointing like Saul the Lord gives us revelation of His Word just the same.
I'm sure the Lord illuminated His people to understand "justification" and the declaration later by Saul that the Law is established in the New Covenant era in the Holy Spirit.
31 Do we then make void the law through faith? God forbid: yea, we establish the law. Rom. 3:31.
Who are the one's that awaited Messiah (Christ)? The Hebrews/Jews.
Who are the first and original Messiah-ites/Christians?
The Hebrews/Jews.
And Jesus added Jews to His Church daily such as should be saved: Jews.
And they remained under the Law under a new dynamic having been born again of the Spirit.