Most things we know we know because “everyone knows” them to be true. They are things we are taught at school and that remain unquestioned in our cultural life. Though much of this “social knowledge” will not be seriously questioned by most of us, we have trained specialists or scientists who will question and test some of it. So we have two types of knowledge: social knowledge and scientific knowledge.
Most of us know figures from the past existed as a form of social knowledge. I know evolution is a fact as a form of social knowledge, and with a little effort I have found I can also know it is true as a more secure, evidence-based form of knowledge.
Most of us know Julius Caesar existed because this is a matter of public record and taught in schools. Specializing students of history know he exists because they become familiar with the evidence: coins with his name and image, busts, books written by him, writings among his contemporaries like Cicero speaking of him. His existence and career is also a very powerful explanation the way Rome and its conquered territories came to be ruled by an emperor.
There is a constellation of other persons in Caesar’s life for whom we don’t have the same strength of evidence. But the fact that those others are written about by authors who express intentions to address the facts of his life gives us strong confidence in the probability of their existence, too.
Some historical persons such as Socrates who have become part of the web of our social knowledge are from time to time questioned by specialist students and scholars. But many of these specialists are satisfied Socrates existed on the strength of the independence of the ancient testimonies. Not only is Socrates found among the writings of his reverential devotees like Plato and Xenophon, but he also appears comedy plays by a contemporary playwright as the but of crude mockery.
So when we get beyond social knowledge, specialist students can uncover the more empirical evidence for the existence of ancient persons. What persuades is where that evidence is multiple, independent and not self-serving or agenda driven.
In the case of Jesus, all the incontestable evidence derives from a single “community”, the Christians themselves. Of course there were factions within that community, but we can still speak of them as a singular ideological community standing apart from the rest of the world. Not only does the evidence derive from a single ideological source, but it is all clearly agenda driven. The references to Jesus are part of theological writings that express clear intentions to persuade readers to submit to the new religion that worships Jesus. (Some scholars will take exception to the claim that the early Christians themselves worshiped Jesus, but the writings we have about Jesus all signal their interest in having readers embrace Jesus as the central focus of their religion.)
We may know about Caesar, Socrates and Christ as a form of social knowledge that superficially gives us a confidence that the historical existence of one is as certain as that of the other. But if we are interested in examining things at another level of knowledge through specializing investigations of the evidence itself, we find the foundations for the each are as different qualitatively as the nature of the evidence for Washington and William Tell. (There is still a flickerings of a lingering debate in Switzerland, by the way, as to Tell’s historical existence.)
It is not enough to say that Jesus was not a public official like Julius Caesar and for that reason we cannot expect the same extent of evidence for him. Obviously we can expect more evidence for a public figure. (Although the gospels indicate Jesus was more well known than many public figures whose records have not survived.)
What is significant is the nature of the evidence. We have on the one hand ideologically independent witnesses that do not testify of the name in a self-serving way, and on the other we have testimony that is ideologically confined and that uses the name to promote an agenda. The difference is as stark as day is from night.
Caveat: This qualitative difference alone does not mean that Jesus Christ was not a historical figure. But it does legitimize the debate.