ON QUORA
What were the major differences between the original Jewish-Christian movement and Paul’s Gentile Churches?

There was no original Jewish-Christian movement. The original Jewish followers of Jesus were not Christian, and they had no church.

The early Jewish followers were called Nazarenes, a word which means “guardians”. They followed the teachings found in the TORAH, and they believed Jesus was the Messiah who would return to save Israel and fix the world. They had no hierarchy - no rabbis, priests, bishops, etc. They had no mission. They spoke to Jews to try to convince them that Jesus was the Messiah. They did not condemn anyone who refused to believe.

Paul had some kind of epiphany, which made him become a Nazarene at first. The Nazrenes only spoke to Jews. However, there were some Nazarenes in the Jewish diaspora who felt that their gentile friends should be given the good news that the messianic era was about to come and that these gentiles should join the Nazarene movement. This meant that these gentile friends would first have to become Jewish.

Paul, who had many gentile friends, agreed with this outlook. But he felt that asking his gentile friends to become Jewish would not work. Many of his gentile friends were Greeks and Romans, and most Romans had no wish to be Jewish. They had no wish to observe dietary restrictions or to waste one day a week doing no work. Their men certainly had no wish to be circumcized, which they regarded as genital mutilation..

He wanted his friends to believe that Jesus was a Messiah, a word which he had to explain to his gentile friends. But in doing so, he had to tell his Roman friends that Jesus was a Jew who would return to earth and destroy their Empire. That made Paul uneasy. He was, after all, a Roman citizen himself.

He was a Jewish Nazarene and a Roman. How could he resolve that dichotomy?

The answer manifested itself easily. Gentiles could become Nazarenes without becoming Jews.

Paul had been a lawman - a policeman, and he valued the idea of law as a guard against chaos. That s what the TORAH was all about. Rules to keep society away from chaos. But Paul knew that many Jews of his time had difficulty keeping all the commandments, even though they lived in a society guided by the commandments of the TORAH. How much more difficult would it be for his gentile friends to keep the commandments? And that would be even if they wanted to keep them, which most of them did not want to do.

Paul was a lawman, dedicated to a life of law but even he had difficulty with keeping commands.

He says to his Roman buddies,

I do not understand what I do. For what I want to do I do not do, but what I hate I do. **16 **And if I do what I do not want to do, I agree that the law is good. As it is, it is no longer I myself who do it, but it is sin living in me. For I know that good itself does not dwell in me, that is, in my sinful nature. For I have the desire to do what is good, but I cannot carry it out. **19 **For I do not do the good I want to do, but the evil I do not want to do—this I keep on doing. Now if I do what I do not want to do, it is no longer I who do it, but it is sin living in me that does it.
Romans 7:15-20

This dramatic struggle within his own nature further convinced him that striving to be a lawful person would lead to ultimate failure on the part of Jew and gentile alike. The more he thought about it, the more assured he became that the TORAH was not the path to follow because

What shall we say, then? Is the law sinful? Certainly not! Nevertheless, I would not have known what sin was had it not been for the law. For I would not have known what coveting really was if the law had not said, “You shall not covet.” But sin, seizing the opportunity afforded by the commandment, produced in me every kind of coveting. Apart from the law, sin was dead. Once I was alive apart from the law; but when the commandment came, sin sprang to life, and I died. I found that the very commandment that was intended to bring life brought death. For sin, seizing the opportunity afforded by the commandment, deceived me, and through the commandment put me to death.
Romans 7: 7–11

What Paul was saying, without actually saying it, is that TORAH is the road to sin.

Jews believe that people commit sins. They are individual acts of rebellion against God. But Paul uses the word “sin” as though it deserves an upper-case S; as if it were some free-floating, amorphous substance into which man can be trapped, and once trapped, man is hopelessly drawn away from the TORAH.

What is his solution?

Who will rescue me from this body that is subject to death? Thanks be to God, who delivers me through Jesus Christ our Lord!
Romans 7:24, 25

Since TORAH will not save him from "sin", only the belief that Jesus is the Messiah (Christ - Greek for Messiah) will save people from death through sinning.

But now, in Paul's mind, "Christ" is no longer the militant Jewish Messiah delivering Jews from their Roman enemies; he is now a divine being, a son of God, who saves people from sin and death by believing in him; accepting his own death as a redemption from the Sin created by the TORAH.

Paul now wants to share his new revelation with his Nazarene comrades. But when he does so, they are appalled by his negative, blasphemous rejection of the holy TORAH. He becomes anathema to them. Word of his apostasy gets back to the Jerusalem Nazarenes and their leader, James (YAKOV), the brother of Jesus, and James becomes unhappy with Paul.

Paul begins to realize that Nazarenes will not accept his ideas about Jesus and TORAH, and if not Nazarenes, then certainly not Jews who are not Nazarenes.

As regards the gospel, they are enemies for your sake. But as regards election, they are beloved for the sake of their forefathers. Romans 11:28

The Jewish People, he says, are not only willfully blind to the gospel (the good news of God's salvation through Jesus' ressurection); they are enemies of that good news; hence enemies of God who sent Jesus. But even so, God loves them because of the merit of their Biblical superhero ancestors. That is to say, they are God's enemies who are loved by God, not through their own instrinsic human worth, but only because they are lucky enough to have Abraham, Isaac, and Jacob as their proto-Christian daddys. But still, they are damned.

Paul begins to look upon his fellow Jews as "lost", beyond salvation. He therefore starts to look for converts only among Gentiles. He becomes Apostle to the Nations par excellence.

He founds gentile congregations in several cities in Greece and Asia called EKKLISÍES, Greek for “churches”.

In the Syrian city of Antioch, people start calling Paul's converts, "Christians", followers of Christ, and thereafter, Paul himself begins to refer to them as Christians. He and they are no longer Nazarenes. A new faith, distinct from Judaism, has come into being.

Back in Jerusalem, James has become aware of Paul's desertion of Israel, and he sends his own Nazarene missionaries to Paul's congregations to convince them to become Jewish Nazarenes. In some places he almost succeeds, notably in the Greek city of Corinth and in the Asian area called Galatia. But the outbreak of a war between the Jews in Judea and the Roman Empire results in the destruction of Jerusalem and the end of the Nazarene leadership.

Seeing that Israel has not been spared from Roman victory, many Nazarenes abandon the sect, and the remaining Nazarenes are deemed heretics by the rabbis and are excommunicated from the synagogues.

Over the next few centuries, the Nazarene movement splinters into separate sects, all of which disappear by the end of the 5th century.

Paul's Christianity continues to grow, becoming the official religion of the Roman world.