Christianity is syncretic - is that not inherently subversive of the source?

Oh I think I see what you mean. To one extent every religion is. No one starts from page 1. I am not quite seeing however what Biblical Jesus borrowed from Rome that the Jews of the area hadn't already. Can you list some examples?

And in this way it created common ground regional cultures, but the direction of the syncretization was also that of Romanization - the new mythos served to legitimize the earthly authority of Rome (and their territorial claims) in a way the teachings of the Jewish tribes had not.

Really only discussing what Biblical Jesus is supposed to have said. He was clear that he was only there for the lost sheep of the Jews, not for the rest.