N.B.: since I am not a crank who believes that the earth is hollow or that absent tin foil on my head the CIA can hear my thoughts please be assured that what follows is written with my tongue quite solidly in my cheek, in order to show why it is that disbelieve in Jesus's existence is intellectually on par with the aforementioned absurdities. It does so through reductio ad absurdum, a wonderful rhetorical device with a splendid pedigree. Now, without further ado...
Let us suppose, for the sake of argument, that Jesus never existed and that the gospels are myths. Now, among the four canonical gospels is one attributed to a man named Luke. Presumably if Jesus is a myth then Luke is too, but we can nonetheless suppose that somebody actually wrote this gospel. Now, there is also another text out there called "The Acts of the Apostles," that purports to be the sequel to the aforementioned gospel by a possibly existent author. Now, whether it was written by the same person or not it seems reasonable to suppose that if the first volume consists but of myth that the second includes little to no history. Yet that volume constitutes our earliest account of Paul's life; indeed, all other accounts of Paul's life are largely dependent on this one. Thus it seems quite likely that if the hero of the first volume, viz. Jesus, was a myth, then the hero of the second volume, viz. Paul, is also a myth.
Now, you might object: but we have letters from Paul! Well, perhaps they are all forgeries, written to sustain the fiction that is Acts. Same with the reference to Paul that is found in 2 Peter 3:15-16. You might object: but we have people referring to Paul in the second century. Well, perhaps those are forgeries too. After all, much of the material that we have from the second century is actually found in manuscripts that date from centuries later; and even if they come from the second century perhaps they are simply part of the deception. Maybe none of those people who cite Paul themselves existed! You don't know; you weren't there. So all we're left with are a few manuscripts dating to the second century that mention a guy named Jesus; a few others that mention a guy named Paul. All told, not enough to affirm their existence.
Yet we don't even have that. What we have are some papyrologists claiming that they have found manuscripts dating from the second century in which Paul is mentioned. How do I know that they are qualified to speak on the matter? And even if they are, how do I know that they aren't in on the conspiracy also? I cannot check for myself after all, I lack access and qualification. I have but their word for it. And I'm such a cautious fellow that I cannot but doubt their testimony on the matter. So really all we have is some twentieth-century scholars, who might have vested interests in establishing the existence of Jesus and Paul, telling us that they have some second-century material that mentions these two figures. And really, I've never met most of these scholars, so I don't even know if they exist. And even if I met them, really I'd have but met people claiming to be these scholars. How could I ever verify that their claims are true and good?
Indeed, how do I know that belief in Jesus or Paul existed before the 20th century? Sure, I read in history books that it was recurrent through history. But those books were written quite recently. They claim to be based upon scholarship, but those scholars might be fictions. They claim to be based upon historical documents but those might be lies. I've not seen most of these documents, after all. I really have no direct experience of belief in Jesus's existence before attending Sunday School in the 1980s. So really I can't be sure that such belief existed before that time. You might tell me that you have memories of such belief from before that but how do I know I can trust you on the matter? Or that you even exist?
Thus do I conclude that no one believed in the existence of Jesus prior to 1983, and that in fact Jesus belief predates Return of the Jedi by just a few months.
Now, let me remove my tongue from my cheek. What I'm done here is intentionally mimic Richard Whatele, who used this sort of radical skepticism to prove that one could not be sure of Napoleon's existence...and this whilst Napoleon was yet alive! Of course Whateley's tongue was in his cheek, because obviously Napoleon existed. But in the process he advances a biting critique: if a line of historical reasoning leads to the conclusion that the greatest news maker of the day never existed then perhaps it is better considered a line of historical unreasoning.
In the end, as with most other conspiracy theories, I'm left wondering: what possible motive would anyone have had for making up Jesus or Paul or the modern New Testament papyrologist? Yes, perhaps with sufficient sophistry I can show how it is logically possible that these figures never existed. But have I given a convincing explanation for why anyone went about this effort? Why bother? Who, in the late first or early second century would have thought it was a great idea to make up Jesus or Paul, if these were not an invention of the 20th century?