At what point did Judaism and Christianity diverge? Christianity and Judaism share the Old Testament (The Hebrew Bible), so it should be safe to assume they are the same people at least during the Old Testament period. Since when did they disagree?
tl;dr> It began in the Council of Jersualem (55 AD), but was cemented by the destruction of the Temple in 70AD.
1. Gentiles were released from Jewish custom
The divergence is clearly articulated in Acts 15 - at the "Council of Jerusalem," often pegged at 50 AD - roughly 20 years after the Crucifixion. Acts 15 sets up the situation as follows:
Certain people came down from Judea to Antioch and were teaching the believers: “Unless you are circumcised, according to the custom taught by Moses, you cannot be saved.” 2 This brought Paul and Barnabas into sharp dispute and debate with them. So Paul and Barnabas were appointed, along with some other believers, to go up to Jerusalem to see the apostles and elders about this question. ... Then some of the believers who belonged to the party of the Pharisees stood up and said, “The Gentiles must be circumcised and required to keep the law of Moses.”
In a nutshell - it's the old circumcision debate: Do converts need to go under the knife?
Prior to this disagreement, Christians were simply Jews that believed Jesus was the promised Messiah. When the Messiah came, however, he expanded His mission in such a way as to afford salvation to the Gentiles. (See Matthew 28:19-20)
Peter speaks to this issue, recalling the fact that the Gentiles were being added to the mission - first in Acts 10, and now again in Acts 15:
6 The apostles and elders met to consider this question. 7 After much discussion, Peter got up and addressed them: “Brothers, you know that some time ago God made a choice among you that the Gentiles might hear from my lips the message of the gospel and believe. 8 God, who knows the heart, showed that he accepted them by giving the Holy Spirit to them, just as he did to us.
That they were included, however, necessarily raised the question of whether or not converts needed to follow all the practices of the Jews. Peter, Paul, Barnabas, and the other apostles were clear that the grace of God implied that Gentiles need not do Jewish custom.
“It is my judgment, therefore, that we should not make it difficult for the Gentiles who are turning to God.
It was even suggested that a letter should be written, in which they said:
Therefore we are sending Judas and Silas to confirm by word of mouth what we are writing. 28 It seemed good to the Holy Spirit and to us not to burden you with anything beyond the following requirements: 29 You are to abstain from food sacrificed to idols, from blood, from the meat of strangled animals and from sexual immorality. You will do well to avoid these things.
By exempting the Gentiles from the rest of Jewish practice, a separate identity began to form.
2. The Persecution of the Jews diminished their importance in the Church, and vice versa
With the destruction of the Temple 15 years later, it simply became much harder to become a Jew. As OzTorah writes:
In the post-destruction Jewish community, re-grouping and greater solidarity moved the Judeo-Christian group to the fringes. The earlier latitude towards separatist and fringe groups had become a luxury, especially when the Jesus party increasingly distanced themselves from their fellow Jews. The Judeo-Christians suffered a diminution in numbers and now, though not without an internal struggle, rebuilt and repositioned themselves as an increasingly gentile group, with new adherents directly coming to the new group without having to go through the old one first. They were not a monolithic community; they included at least four sub-groups (Ebionites A, Ebionites B, Nazarenes, Gnostic Sycretists and Elkesaites) – but they, like the Jews, needed to find sufficient unity to plan a secure future.
After much internal debate it became possible for an outsider to become a Christian without ever being part of Judaism, either through genealogy or choice. Could you be a Jew without the Sabbath, festivals, circumcision (Jews were not the only ancient people to view uncircumcision as shameful) and dietary laws? The answer was no – but you could become a Christian. Could you be a Jew without saying Sh’ma Yisra’el – “Hear, O Israel” and proclaiming the absolute invisibility and indivisibility of God? – again no: but you could become a Christian if you accepted the re-worked status of Jesus (developed and taught by Paul and his supporters though not necessarily required by a reading of Jesus’ own words) as messianic and part of divinity.
Sharper language than ever before began to be used in Judaism – it was a time of crisis when it was necessary to know where people stood – and heretics could no longer be treated with kid gloves. About the end of the century the Synagogue liturgy introduced a prayer which came to be known as Birkat HaMinim, the Blessing (Against) the Sectarians. ...
There was no authoritative decision to expel the Christians from Judaism but their exclusion came about gradually. The gentile Christians never were part of Judaism. The Jewish Christians still met halachic (Jewish legal) identity criteria but were excluded from officiating at Jewish worship because they regarded Birkat HaMinim as directed against them – whatever its motives at the time of its formulation – and their books were deemed to lack sanctity. In consequence they felt increasingly unwelcome. Christians were still found in the synagogues at least until the time of Jerome in the fifth century CE. In the second century CE Justin Martyr agreed that Jewish Christians who continued to follow Jewish usages were still to be considered “brethren” but as time went on, pressure was exerted to discourage the practice of Judaism by Christians.
The final break was due to the Romans when Jews (including Jewish Christians) were prohibited from entering Jerusalem; the re-established Jerusalem Church was thus an essentially gentile one.
With this polarization, rapid growth outside of Israel (Paul's ministry, in particular, was centered in Turkey and Greece, as evidenced by the fact that Corinth, Galatia, Ephesus, Phillippi, Colossae, and Thessaly all got their own books of the Bible!), and the persecution of the Jews inside of Israel, led to an imbalance of population.
Basically, the "Jews" dwindled and the "Gentiles" grew. That, coupled with the freedom from the practices of the Jews eventually led to a situation where the church's early identity divested itself of its Jewishness.
3. By the second century, there was hostility
With the firm acceptance of Paul's letters by the end of the first, the anti-Judaizer faction became well entrenched. By the middle of the second Century, some heretics like Marcion were already trying to remove any vestigial Jewish influence. When Marcion produced his first canonical list, for example, the entire Old Testament was relegated to non-canonical status. (Note - Marcionism is considered heretical nowadays, and rejecting the Old Testament is considered heretical, but I point it as way of evidence that the split had already occurred.)
By this time, "Christ-killer" rhetoric and the like, along with regional chauvinism prevailed, and enmity between the Jews and the Christians was mainstream. Modern Judaism took root in the Pharisitcal and Essene movements and followed its own course thereafter.
That's fairly straightforward.
Jesus Himself was Jewish. The divergence came from Him. Whether it was at His death, burial, and resurrection, immediately afterward, or when He began His ministry is up to interpretation and debate, but it was Jesus, and the New testament (New Covenant) established by Him that marked the split.
Those who recognized Jesus as the promised Messiah were the first Christians. Perhaps they could have been considered Messianic Jews, but since Christianity is defined as following Christ, following Him is the logical determining factor as to whether or not one is a Christian. Jews who followed him were, for all intents and purposes, the first Christians, and that's where the divergence lies.
As David has pointed out, but without detail, the division was with Jesus' life, death, and resurrection. There was nothing that could be called Christianity before Jesus.
During the last three years of His life Jesus gained many followers, however, he never made any deliberate attempt to divide the Jewish faith, nor change it.
17 “Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them. 18 For truly I tell you, until heaven and earth disappear, not the smallest letter, not the least stroke of a pen, will by any means disappear from the Law until everything is accomplished. Matt 5:17
These are Jesus' own words. He was claiming that the Law (the ancient Hebrew texts) was not to be removed by his coming, but to be uplifted and proven and fulfilled. Jesus was claiming to be a fulfillment of prophecy. The Gospels (Matthew, Mark, Luke, and John) detail Jesus' ministry during the last three years of his life, where he attempted to fulfill Jewish prophecy as the Messiah or the Christ.
However, the Jews of the time rejected Jesus and had Him executed, but that was part of the prophecies. He was disgraced and his followers dispersed. This is also detailed in the Gospels.
Later, however, Jesus' disciples and a few other persons began claiming to have seen Jesus alive. Not alive in that He actually survived execution, but alive in that God had raised Him from the dead and glorified Him. Over the course of a short time many more claim to have seen the resurrected Jesus until He ascended into heaven 40 days (I think) after His resurrection. This too is detailed in the Gospels.
Most theologians would say that it is was this point that Christianity split from Judaism. There were still many Jews who would not believe the resurrection, therefore, there was nothing miraculous about Jesus, but many others did believe, therefore, He became known as the Messiah and the Christ, the One who fulfilled prophecy. Secular historians must neglect the resurrection as fact, however, they do not neglect that Christianity was recognized as from Judaism but different and distinct from Judaism quite early; most estimate that Christianity was well established as a separate religion before AD 200.
According to the Bible the word Christian, which means literally 'small Christ' and means figuratively 'strives to be like Christ', was used very soon after the Resurrection.
The disciples were called Christians first at Antioch. Acts 11:26
The Book of Acts was written to cover the events immediately after the Resurrection to show how Christianity came to be such a large movement then, which continues today.
Originally, Christianity, as taught by Christ (Jesus), was meant to support and augment Judaism. The Christ came to fulfill the Law. Christianity became something separate from Judaism very shortly after Jesus' Resurrection. The disagreement was whether He was the Christ or not. Christians believe He is and the Jews believe He was not. Because there were many on both sides the two factions were made. The factions persist today for the same reasons, generally speaking.
In the view of those who follow Christ, Christianity is a continuation and fulfillment of the Judaism of the Old Testament. A certain part of Judaism diverged from that lineage most prominently at about noon on the day Jesus was crucified:
14 It was preparation day for Passover, and it was about noon. And he said to the Jews, “Behold, your king!” 15 They cried out, “Take him away, take him away! Crucify him!” Pilate said to them, “Shall I crucify your king?” The chief priests answered, “We have no king but Caesar.” 16 Then he handed him over to them to be crucified.
Many other Jews, however, did recognize and follow the awaited Messiah. For example, the Letter to the Hebrews was directed to Jewish Christians. This is what the word "Christ" means: Messiah. So "Messianic Judaism" is effectively just "Christianity".
Alternatively, the point at which this divergence occurred was the moment when Jesus was conceived by the Holy Spirit, or when the first or last of the various Messianic Prophecies was fulfilled.
Judaism only began with the completion of Babylonian Talmud in early sixth century AD.
The completion of Babylonian Talmud was around 500 AD (Source - Book "Alfred Edersheim: A Jewish Scholar for the Mormon Prophets" by Marianna Rica, Page 44).
Let me give evidences from the Jewish rabbis themselves.
Jewish Rabbi Stephen Wise (Formerly Chief Rabbi of United States) said, "The return from Babylon and the adoption of the Babylonian Talmud, marks the end of Hebrewism and the beginning of Judaism." (Source -Book "The F.O.J. Syndrome in America: Bamboozled Americans and their vile brainwashers", Page 253-254, American Mercury, Volume 6).
"The Bavli [Babylonian Talmud] has formed the definitive statement of Judaism from the time of its closure to the present day." — Rabbi Dr. Neusner (Source - Rabbi Dr. Jacob Neusner, quoted by Norman F. Cantor, The Sacred Chain (A History of the Jews), page 112).
Many people mistakenly think that Judaism is the religion of the bible. This is a fallacious impression. Judaism is not the religion of the bible.
On page 59 of “Judaism and The Christian Predicament,” Rabbi Ben Zion Bokser says: “This is not an uncommon impression and one finds it sometimes among Jews as well as Christians; that Judaism is the religion of the Hebrew Bible. It is, of course, a fallacious impression. Judaism is not the religion of the Bible."
Talmud being the holy text of Judaism can be found in the Talmud itself.
Erubin 21b (Soncino edition): “My son, be more careful in the observance of the words of the Scribes than in the words of the Torah (Old Testament)."
Judaism is a reconstruction out of Pharisaism.
“Pharisaism shaped the character of Judaism and the life and thought of the Jew for all the future." (Source - The 1905 edition of the Jewish Encyclopedia says under the subject of “Pharisees”).
The Pharisees and the scribes obeyed Traditions of the Elders which nullified the word of God (Matthew 15, Mark 7). That's why Jesus Christ's biggest opponents were the Pharisees and the scribes (Matthew 23, Luke 11).
Traditions of the Elders evolved and finally came to be called Babylonian Talmud which marks the beginning of Judaism as mentioned above.
This is also agreed by Jewish society.
Rabbi Michael Rodkinson - "The Talmud, then, is the written form of that which, in the time of Jesus, was called the Traditions of the Elders, and to which he makes frequent allusions." (Source - The History of the Talmud, Vol. II, page 70, Chapter IX).
Rabbi Herford, in Jewish Quarterly Review, vol 2 (1890) (pp. 454-464) mentions that Oral Torah became Traditions of the Elders and finally to Talmud.