I’ve long been perplexed by the frequent complaint from mythicists
(i.e. those who claim that Jesus was a purely invented figure, not even based on a real historical human individual) that those working on the historical Jesus simply assume as a presupposition that Jesus existed, rather than addressing the question directly.
I think such individuals are looking for a demonstration by historians, in the introductory part of their book about Jesus, “proving” he existed, before going on to discuss anything he may have said or done. That this is what is meant seems clear because one may cite a saying or incident that is generally considered authentic, only to be met with the retort, “But how do you know he even existed?”
Such objections reflect a serious misunderstanding of the historical enterprise. I think it is safe to say that there is no historical figure from the past that we know existed apart from evidence for actual things he or she said or did. We know George Washington existed because he wrote documents, because he served as President of the United States, because he slept here or there. There is no such thing as proof of a historical person’s existence in the abstract or at a theoretical level. There is simply evidence of activity, of speech, of things said or done, of interaction with others.
And so when historians engage in the tedious but ultimately rewarding process of sifting through the relatively early texts that mention Jesus, and painstakingly assess the arguments for the authenticity of a saying or incident, they are not “treating the existence of Jesus as a presupposition.” They are providing the only sorts of evidence we can hope to have from a figure who wrote no books or letters, ruled no nations, and did none of the other things that could leave us more tangible forms evidence.
And so I will state once again what is obvious to historians and New Testament scholars but apparently unclear to some who are not entirely familiar with how historical investigation works. Historians are confident Jesus existed, first and foremost, because we have sayings attributed to him and stories about him that are more likely authentic than inauthentic. We have enough such material to place the matter beyond reasonable doubt in the minds of most experts in the field. And in order to deny that Jesus existed, one has to posit conspiracies and misunderstandings which, if one is willing to entertain such scenarios, could effectively be used to deny the existence of just about anyone in history. And even in the case of the most plausible mythicist scenario (not that they ever take the time to make a positive case for how the myth was invented and how it came to be misunderstood so quickly as being about a historical figure) we never get a scenario that is more probable than one that regards there as having been a real historical figure Jesus, however much he may have been obscured by later developments and dogmas.
And so, in short, the existence of Jesus is not something that can be proven in the abstract. This is simply stating the obvious, and is true of any historical figure. And the reason all mainstream historians and New Testament scholars believe Jesus existed? Because they have found at least one thing that he is purported to have said, or done, or had done to him, that seems very likely to be authentic. And if there is an authentic saying of Jesus, or action by him, or he was crucified, then he existed, because there is no such thing as an authentic historical action by a non-existent person.