I don’t know historically speaking how much we can establish about Christianity before Paul.  But based on the fact that matters relating to Mosaic Law, the status of Gentiles were settled only after Jesus’ earthly ministry, and the fact that his closest followers still expected the restoration of Israel, it’s probably safe to say that whatever the very early movement looked like, it was fundamentally a Jewish movement. I usually describe Jesus as a Jewish reformer. 

I think one can argue that Paul’s influence challenged the beliefs and identity of Jesus’ earliest followers, but I tend to view Paul as the eschatological accelerator. Jews had always thought that Gentiles would fund righteousness through Israel’s repentance. Paul reversed the order. He perhaps thought he was preparing non-Jews to meet the Jewish God in the Jewish End of Days. 

I agree the Ebionites probably reflected an early stage of the movement, but I wonder what made them “Christian”?  Apart from believing Jesus to be Messiah, they were essentially Jews like other Jews of their time.  I would even argue that the Ebionites, and more precisely the community based in Jerusalem whose members included Jesus’ family and apostles, likely reflected most accurately the core of Jesus’ teaching. They personally heard Jesus speak. They remained Jews as they did not understand Jesus to have taught them not to continue in Judaism. The Christian identity I think developed outside of this community. As Acts tells us, the first people to be called Christian were in Antioch, not Jerusalem. 

To learn some differences and similarities of the Nazarite or Nazoraean religion, spelled Nazarene or from Nazareth that, in part, became Christianity, look up “Mandaeans” or “Mandaeanism”. The Mandaeans are the descendants of John the Baptist. John the Baptist (Yahia Yohanna) is their most important prophet. The Mandaeans are called living Gnostics or the last Gnostics.