"The 'parting of the ways' has in recent years become a standard model for describing the split between Judaism and Christianity" (Judith Lieu). James Dunn, however, claims that the split wasn't between Christianity and Judaism, but between Gentile and Jewish Yeshua followers.
The debate was not, “should I be Jewish or Christian?” but...
Should a Jewish Yeshua follower still practice the ceremonial requirements of Torah (namely kashrut, circumcision, Shabbat, and the Feasts), and
Is Christianity still connected to Judaism/The God of Israel?
Some see this 'parting' in the late first century. Judaism emerged from the disastrous Great Revolt in 70 CE as Rabbinic Pharisaical Judaism under Ben Zacchai (who made a deal with General Vespasian to escape Jerusalem and form a school in southern Israel). This is also possibly the time when the 19th benediction against Nazarenes was added, whereby a Jewish believer in Yeshua would have to pray curses on himself when praying at the synagogue, thus rooting Messianic Jews out. They see this as when Judaism broke from Christianity and vice versa.
But Mark Kinzer points out in “Post Missionary Messianic Judaism” that within the ekklesia (church), they were arguing about Jewish life within the life of the Jewish believer and the non-Jewish believer still in the late fourth century.
Scholar Phillip Cunningham claims that...
"Many Christians continued to be strongly attracted by Jewish traditions. This fascination not only led these Gentiles to adopt some Jewish customs in an informal way but also encouraged a significant number of [believers]—both Jews and Gentiles—to continue some degree of Jewish practice within church communities.
"It appears that for centuries some Christians would frequent synagogue functions as well as local church liturgies. The boundaries between the two groups remained quite porous for a considerable period (long after the first century)."
Lindemann, Albert S.; Levy, Richard S..
Antisemitism (p. 57). OUP Oxford. Kindle Edition.
This was problematic for many of the bishops who were threatened by the continued fascination with Jewish life by their constituents. This led to some of the most ruthless diatribes from jealous leaders against the Jewish people. The orators took passages from the New Testament that, in context, were interfamilial debates and reimagined them in a way that made Yeshua (a faithful Jew, observant in every way) antisemitic.
Rebukes against corrupt leaders were made to appear as rebukes against Jews in general. Remember, every follower of Yeshua was Jewish, and He was beloved by the masses. They arrested Him in secret because "they feared the throngs" of Jews! (Matt. 21:46)
Cunningham continues, "Taking polemical passages from the New Testament, which had mostly arisen as part of an inner Jewish debate, and reading the internal criticisms of the Hebrew prophets as evidence of constant Jewish failures, Christian teachers attacked Judaism’s respectability,” with extreme rhetoric.
In the third century, the scholar Origen accused the entire Jewish nation of "conspiring against the Savior of the human race."
One of the most antisemitic, demonic diatribes against the Jewish people came from John Chrysostom in Antioch in his series of sermons “Against the Jews” (that was the title!). Where he claimed, amongst other things, that it was every Christian's duty to hate the Jews. Here is an excerpt:
The synagogue is worse than a brothel…it is the den of scoundrels and the repair of wild beasts…the temple of demons devoted to idolatrous cults…the refuge of brigands and debauchees, and the cavern of devils. It is a criminal assembly of Jews…a place of meeting for the assassins of Christ…a house worse than a drinking shop…a den of thieves, a house of ill fame, a dwelling of iniquity, the refuge of devils, a gulf and an abyss of perdition…I would say the same things about their souls… As for me, I hate the synagogue…I hate the Jews for the same reason.”
He prohibited his members from going to the synagogue. Why? Because many were going to the Jewish synagogue in hopes of learning something valuable. While Chrysostom was antisemitic, clearly, some in his parish were philosemitic (having an appreciation for the Jewish people and their history).
If the Jewish/Christian split—the parting of the ways—had already taken place by the end of the first century, why are they still talking about it 300 years later?
Clearly, many believers, Jews and non-Jews, favorably viewed the Jews and their customs.
We know that up until 325 CE, many in the Church still used the Jewish Passover to remember the death and resurrection of Yeshua. That was one of the main reasons for the Council of Nicaea. This was part of Emperor Constantine's letter at the Council's conclusion.
"It was declared to be particularly unworthy for this, the holiest of all festivals, to follow ... the Jews, who had soiled their hands with the most fearful of crimes, and whose minds were blinded. We ought not, therefore, to have anything in common with the Jews, for the Saviour has shown us another way..."
Some bishops, like Victor of Rome in 190 CE, were so threatened by this that they wanted to excommunicate any believer who celebrated the death and resurrection of Yeshua on the Jewish Passover. It would be debated for another 135 years before the Bishops of Nicaea would outlaw it, showing that many Christians continued to celebrate the Passover. Why outlaw it if no one was observing? The Church Council only dealt with relevant issues.
One more example. In 306, in Elvira, Spain, a church council forbade Christians to eat with Jews or allow them to bless their crops. Why? Obviously, this was taking place at a level that had the bishops alarmed.
What began as an internal schism within the ekklesia became "the schism ... between the multinational ekklesia and the Jewish people" (Kinzer, 211). In other words, before the Church completely split from Judaism, she first split from her Jewish constituents. To paraphrase Jerome of Striden, a fourth-century theologian, "He who would be both Christian and Jew, can be neither Christian nor Jew,"—a mantra that both mainstream Christianity and Judaism would come to embrace.
Thus, for healing to take place, Christianity is not merely seeking reconciliation with Judaism but with Jewish Yeshua followers who had to deny their Jewishness to embrace the Jewish Messiah.
Kinzer quotes Thomas Torrance, "The deepest schism in the one People of God is the schism between the Christian and the Jewish Church" (Kinzer, 212) — meaning Gentile believers and Messianic Jews.
You can't treat the result of the disease without dealing with the root of the disease.