The majority of historians agree that Jesus was an actual person who was probably born around 4 BCE. He was raised as a Jew, and when he began preaching, his intention was not to initiate a new religion but to reform Judaism. Because his message highlighted the imminent coming of the kingdom of God, he was seen as an end-time or apocalyptic prophet. The followers of Jesus did not become known as Christians until long after his death. They were thought of as a Jewish sect, of which there were many at the time, such as the Sadducees, the Pharisees, the Zealots, and the Essenes. Historically, it was the Romans who arrested and crucified Jesus, possibly for inciting disturbances in crowds.
Even when the followers of Jesus began to spread his message after his death, they focused on Jewish communities in Palestine and other parts of the eastern Roman Empire. When gentiles began to join the communities of Jesus's followers, they were at first expected to conform to certain Jewish laws and customs.
The man who was originally called Saul of Tarsus but after conversion took the name of Paul is largely responsible for the split that occurred between Jews and Christians. He wrote a series of letters justifying salvation by grace, claiming that circumcision and other Jewish practices were not necessary for salvation. This put him and most Christians at odds with Jewish theological leaders. His stand that gentiles as well as Jews could be recipients of salvation alienated Jews intent on keeping the Jewish law, but it enabled the spread of Christianity to gentiles throughout the Roman Empire.
To Jews, Christians became known as Nazarenes and were regarded as a completely different religious group. This meant that Jews who became Christians would no longer be regarded as Jewish. This was a prohibitive step to take unless someone was truly convinced of the reality of Christian beliefs. The split became so obvious that even the Romans began to treat Jews and Christians differently in a legal sense. For instance, Christians did not have to pay certain taxes levied upon the Jewish people. On the other hand, Judaism was recognized as a religion by Rome, whereas Christianity was not recognized until the Edict of Milan in 313 CE. In 380 CE, with the Edict of Thessalonica, Nicene Christianity became the official religion of the Roman Empire.