Biblical scholar Bart Ehrman argues that there are many versions of Christianity, and there always have been since its inception. What you are representing as the "core teaching of Christianity" is the version that was primarily articulated by Paul, and that became the orthodox version due to its eventual adoption as the religion of the Roman Empire. But even at the time of Paul there were rival interpretations. There were the Gnostics, the group led by James in Jerusalem, and even those who insisted one had to adopt Judaism to follow Jesus. And significantly, Jesus himself probably wouldn't have recognized the interpretation of Paul.

And looking beyond early Christianity, one can pick any period of Christianity's history and find numerous rival doctrines.