Jesus did not set out to create a different religion. His message was of a focus on the fundamentals of Judaism: trust in God and love your neighbour. He spoke against what he saw as the excessive focus on ritual and rigid interpretation of Law at the cost of love. The only really un-Jewish parts of his message were about him being the son of God and the sole conduit between the people and God.
Remember, Jesus was Jewish, all his followers were Jewish and all their worship and practice was Jewish. Jews do not proselytise to not-Jews, so the followers of Jesus spread their philosophy only among their fellow Jews. But after Jesus died, Paul of Tarsus proselytised to non-Jews, and created the first non-Jewish followers of Jesus. Jews did not accept these followers of Jesus as Jews, which would have caused tension. That was the first separation or branching. The Romans did not differentiate between them.
For the next 100 years after Jesus' death, the Romans persecuted the Jews in Israel, destroying the Temple and taking tens of thousands of Jews of all denominations, including followers of Jesus, into slavery outside Israel.
Christianity flourished in exile. It focused on a better world to come, and de-emphasised the Jewish rituals which would have been very hard to adhere to. This was the second separation, such a large separation that Christianity could no longer be considered a branch of Judaism.
By the time of Emperor Constantine, another 150 years later, Christianity was firmly established in the Roman Empire, and the various Christian authorities were making deliberate policies to distance Christianity from Judaism: eg changing the day of the Sabbath from Saturday to Sunday, forbidding commemoration of the Last Supper from being held on Passover and changing the religious calendar from the Jewish calendar to the Roman calendar. Constantine did not believe in either religion, but he was politically canny and saw a monotheistic religion as a way of strengthening his empire. He vacillated between the religions, gradually coming down more and more on the Christian side, and converted on his deathbed.
That was it. Christianity was established as the religion of the Roman Empire, the religion of politics and authority. At the sane time denigration of Judaism was established, to ensure Jews could not derive any power from and confusion with their religion’s younger brother.
And the rest, as they say, is history.