This is a long post, so here’s a summary but please go through it you won’t regret
[ul]
[li]Jesus was thoroughly Jewish, kept the Torah, and never taught/discipled gentiles[/li][li]The earliest Christians were all Torah observant Jews and saw Jesus as their Messiah[/li][li]Belief in Jesus’s resurrection led to different interpretations of the relationship between Jesus and God by Jewish and gentile Christians[/li][li]Jewish Christianity, which emphasized keeping the Torah (among other ideas), came into conflict with Greek and Latin Christianity, which discarded it[/li][li]Greek/Latin Christians prevailed in these conflicts, and got to define orthodoxy for all future Christians[/li][li]Roman power in league with orthodoxy saw to the diminishment and eventual annihilation of Jewish Christianity[/li][/ul]
Nicene Christianity had dramatically different theology than the original teachings of Jesus. This is not to say that what went on to become the orthodox view for almost all modern Christians was not held earlier than the Council of Nicaea (indeed, many proto-orthodox writings survive in the New Testament), but that it was one group among many in the first 3 centuries of Christian history. The outcomes of the major councils of the 4th century, with Roman support, resulted in substantial consolidation of the bishops’ majority view as orthodox and the suppression of diversity within early Christianity.
To understand what the very first Christians probably believed, we have to start with Jesus’s context in Roman Galilee/Judaea in the early 1st century. Jesus seems to have been a pious Jew who lived in the immediate aftermath of the fall of the Hasmonean kingdom, in a period when Jews were grappling with the loss of their short-lived independence and the meaning of God’s promises in their scriptures. To the Jews, the Roman Empire’s newfound rule in the historical land of Israel presaged a return to the aggressively encouraged paganism they had struggled against under the Seleucids.
It should not be surprising that many many Jews placed their hope in a coming apocalypse where their oppressors would be cast aside and a descendant of David would once again sit on the throne. During the Seleucid occupation of Israel through to after the Maccabean revolt, several prominent texts of Jewish apocalyptic literature were written, including the books of Daniel and Enoch. These books were particularly influential on the Jews of Jesus’s time in the early Roman occupation. Jesus quoted from the book of Daniel in several instances in the Gospels, and other New Testament authors quote from or allude to Enoch (e.g. Jude). The language Jesus used like Son of Man, his message of the coming kingdom of God, and his persistent references to the imminent end of the present age were steeped in this apocalyptic tradition, and it is for this reason and others that the scholarly consensus is that Jesus can be principally understood as an apocalyptic prophet roughly in the tradition of Daniel.
In addition to being very concerned with the coming kingdom of God and end of the age, Jesus was deeply committed to the Torah. Though he had some contrarian views (for instance on the meaning of the Sabbath), he emphasized the importance of keeping the whole Law perfectly and said it was the only way to enter the kingdom of God. Jesus never taught gentiles and did not seem to have any inclination to extend his audience to non Jews, even though there were several Greek and Roman cities in Roman Palestine in Jesus’s day.
During his lifetime, Jesus’s followers very well might have seen him as the fulfilment of the messiah prophecies in their scriptures. Jesus himself might have claimed the identity of Jewish Messiah, evidenced by the fact Jesus was eventually crucified by the Romans as the alleged king of the Jews. Whether or not Jesus or his disciples actually made this claim, Rome saw him as a sufficient threat to domestic tranquility to make an example of him.
Whatever was the case during his life, Jesus’s followers certainly came to understand him as the Messiah after his death, and very quickly came to believe God had raised him from the dead. This is where things get interesting. Several of Jesus’s disciples had visions or experiences of Jesus appearing to them alive and immediately began exploring the implications of what that meant in light of what Jesus taught and what they understood about resurrection from their scriptures and the broader cultural milieu. Many of them came to an understanding that Jesus was adopted by God at his resurrection and exalted to be the heir to the dominion of God on earth. Later Jewish Christians pushed the adoption of Jesus by God back to his baptism, and others pushed it back further still to his birth.
Over the course of 20 years after the death of Jesus, other views developed like that of the Apostle Paul that we see in Philippians 2. In this passage (and elsewhere in Paul’s writings), we see indications he thought Jesus existed before his birth and was already in some sense “in the form of God.” But that isn’t quite the same as saying he IS God, since Paul says Jesus was exalted to a yet higher level after his death (and God is already necessarily exalted above all). By the time we get to the gospel of John 40 years later, some Christians had come to believe that Jesus was fully co-equal with God and indeed was God (John 1:1-4). Though this isn’t the full Trinitarian formulation that became accepted later, Johannine Christianity was much closer to the that formula than what we see in the synoptics and in Paul.
At the same time that early Christologies were emerging, Greeks and other gentiles were converting to the faith and starting to come into conflict with Jewish Christians. Paul himself was a Jew but saw himself as an Apostle sent by Jesus to preach to the gentiles. He saw the Torah as a fundamental barrier to Greek inclusion in the family of Christians and spent his career in a delicate balancing act between playing down the importance of the Law when speaking to the gentiles and criticising other Jewish Christians like Peter for not following the Law closely enough (Gal 2). Paul might have had toes in both the Jewish and Greek worlds but later readers of Paul’s letters were overwhelmingly non Jewish and grew to be distrustful of the Jews, even blaming them for the murder of God, leading to the long history of anti-Judaism in the church. While extremists like Marcion (who wanted to excise all the Jewish Scriptures from the Christian canon) didn’t win the day, the way later Christians read their Bible belittled the Jews for not understanding their own scriptures pointed to Christ all along and that the Gospel had superseded the Law, making it obsolete. As Greek and Roman ideas worked their way into the Christian formulations of the triune nature of God and the immortality of the soul, the chasm between Jews and gentiles who both claimed to follow Christ got wider and wider.
Jewish Christians, emphasising the oneness of God, largely kept the early adoptionist view of Christ that developed soon after the resurrection accounts and didn’t adhere to the high Christology that had come to dominate the Greek and Latin church. They kept the Torah and believed gentiles who wanted to become followers of Christ needed to convert to Judaism, including the mandate of circumcision. Because of all this, they would go on to be marginalized and persecuted by the Greek church and denounced as heretics by church fathers like Tertullian (he called them Ebionites). On the flip side, Rabbinic Jews had grown understandably antagonistic to the Christians for the persecution and vitriol they encountered, so any Jews who wanted to follow Christ and belong to a community that would accept them would be expected to renounce Judaism entirely and identify exclusively as Christian. By the 7th century, the oldest traditions in Jewish Christianity were more or less completely wiped out, with orthodox Christianity finally (though not for much longer) substantially united from East to West. In perhaps the ultimate irony, the last remnants of the Jewish Christian tradition might have had encounters with the first Muslims, influencing the Islamic view of Jesus and leaving one final indelible mark on history.