When did Christianity stop being Jewish?

Acts has a party insisting "unless you are circumcised after the custom of Moses, you can’t be saved", and a meeting in Jerusalem deciding otherwise: to "lay no greater burden on you than these necessary things", a short list of abstentions.

Notice what that is. It is a Jewish movement deciding the terms on which non-Jews may join it. Nobody in the room is proposing to stop being Jewish.

In Galatians, Paul gives his own account: an agreement that he would go to the Gentiles and Peter to the circumcised, and then a public confrontation at Antioch. "I resisted him to his face, because he stood condemned." Peter had been eating with Gentiles and drew back when people came from James.

A dispute about who may eat with whom is a dispute about whether this is one community. The letter shows it unresolved and bitter, which the tidier account in Acts does not.

Justin’s Dialogue with Trypho, written around 160, is a Christian arguing with a Jew as with an outsider. Circumcision, in his reading, was given as a mark of judgement.

The rhetoric is already hard. The same chapter says the desolation of the land, and the ban on any Jew going up to Jerusalem, came "in fairness and justice, for you have slain the Just One". Whatever the first century was, this is two things, and one of them is writing against the other.

The older account had a clean break, set going after the destruction of the temple in 70 by rabbinic measures at Yavneh such as the Birkat ha-Minim, the blessing against heretics, and complete by the Bar Kokhba revolt. Steven Katz argued in 1984 that even that blessing "did not signal any decisive break between Jews and Jewish Christians", and a collection titled The Ways That Never Parted asks whether there was a definitive parting at all. The evidence runs late. In the fourth century a church council at Laodicea was still legislating against Christians who rested on the Sabbath or shared in Jewish feasts. It put judaizing under anathema, which is not a canon anyone writes about a practice nobody has.

Read that way, the separation happened at different speeds in different places and was pressed hardest by those with authority on both sides, who wanted a line that ordinary people kept crossing.

Candidates include the admission of large numbers of Gentiles who never kept the law, the destruction of the temple removing a shared centre, the Jewish revolts against Rome and their consequences, and the hardening of both traditions into forms that defined themselves against the other. The collection cited above begins at the Bar Kokhba revolt, after the temple had fallen and Gentiles had come to outnumber Jews in the movement, and which of the candidates did the work is not something this page has read an answer to. A Vatican commission in 2015 named two: Gentiles becoming the majority, and polemics about Jesus sharpening within the Jewish community.

They are not alternatives so much as a sequence nobody planned.

Groups who followed Jesus and kept the law existed for centuries. Jerome describes one, the Nazarenes, as "condemned by the Pharisees", and says of them that "while they desire to be both Jews and Christians, they are neither the one nor the other". Eusebius describes the Ebionites keeping the Sabbath and the law and using a gospel of their own, and in the same chapter explains that their name signifies poverty of understanding.

They are the direct evidence for how long it was possible to be both, and in both of these accounts someone else is speaking for them.

The argument did not stay an argument. A Vatican commission, quoting John Paul II, wrote in 1998 that readings of the New Testament about the Jews’ "alleged culpability have circulated for too long, engendering feelings of hostility towards this people". The same document says that anti-Judaism "in some Christian quarters" led to "a generalized discrimination, which ended at times in expulsions or attempts at forced conversions", and that in times of crisis the Jewish minority "was sometimes taken as a scapegoat and became the victim of violence, looting, even massacres". Dabru Emet, a Jewish statement of 2000, puts it without qualification: "Without the long history of Christian anti-Judaism and Christian violence against Jews, Nazi ideology could not have taken hold nor could it have been carried out."

In 1965 the Second Vatican Council stated, for the Catholic Church, that the passion "cannot be charged against all the Jews, without distinction, then alive, nor against the Jews of today", and that the church "decries hatred, persecutions, displays of anti-Semitism, directed against Jews at any time and by anyone". The date is part of the information.

The Second Vatican Council says the church’s faith and election begin "among the Patriarchs, Moses and the prophets", and a 1987 paper of the Presbyterian Church (U.S.A.) that the church "has been engrafted into the people of God". But for centuries the dominant way of claiming Israel’s inheritance was replacement. A Vatican commission wrote in 2015 that supersessionism gained favour among many of the church fathers until, in the Middle Ages, it was "the standard theological foundation" of the relationship with Judaism. Since 1965 the Catholic Church, and a number of Protestant bodies, have said instead that God’s covenant with the Jewish people endures.

One Jewish answer is that Christianity is not a form of Judaism. Dabru Emet, a statement published in 2000 over the names of more than 170 rabbis and Jewish intellectuals speaking for themselves, respects it as "a faith that originated within Judaism" and says: "We do not see it as an extension of Judaism."

Those are not symmetrical positions, and there is no neutral vantage point from which to referee them. What a reader can see is that the first generation would have found the question strange.