An interesting thing about what we have now in Christianity though is that it basically spawned as-is in the first century, with the Gospels and Paul’s letters being written decades apart, as well as Church Father’s writings being very consistent. Main differences has been the Roman Catholic Church developing their own doctrine such as Purgatory over time, while the Orthodox and the Protestants tend to reject such developments.
So if it is just a legend, something must have happened to cause a consistent story to develop fairly quickly in comparison to the likes of other legends.