Religion

As far as proof contained within the bible itself, you have different chapters written by different people in different languages all referencing the same person. While some of these chapters were undoubtedly sourced from the same lost reference, some were not. The weak point is at best it's 4th or 5th person accounts written decades if not centuries after this alleged person was long gone.

Outside the bible there's a couple of 3rd person (at best) accounts of a person named Jesus, although there's almost no testament to his ministry other than his name and manner of execution.

The fact that there's almost no Roman references to Jesus highly suggests he was insignificant in his own time, at least to the Romans.

It's easier to believe there was a person named Jesus who was probably a Rabbi over a non-mainstream sect of Judaism. It's not as if there would have been a shortage of such people from which a mythology could be built upon. The question is how closely the mythology matches the actual person, and it's much harder to believe they were all that close. Decades and centuries of oral tradition don't do much for fidelity of a story, particularly when much of it was made up from the very beginning.