The Big Split - Fall of Jerusalem
The fall of Jerusalem to the Roman armies in 70 C.E. was a crucial event for all of Judaism, including the followers of Jesus. The Jews had to redefine Judaism after the destruction of the temple, which had been the center of their practice. This was true for the Jesus-believers as well. The book of Hebrews is apparently an attempt to make sense of Jesus-believing Judaism without a temple.
With the destruction of the temple, the Saducees, who oversaw the temple functions, ceased to have any significant influence. So it fell to the Pharisees to define what Judaism would be in the absence of a temple. From this developed Rabbinic Judaism, which focussed on synagogues and Torah study, including the teachings of the rabbis.
Many of the followers of Jesus had left Jerusalem in the early days of the siege of Jerusalem, remembering Jesus' words in Matthew 24:16-21. They took up residence in Pella. The Jews who were left in the city felt justifiably abandoned, and another reason was had to despise those who left.
The Big Split - Blessing against Heretics