First of all, about theheno's contrast between Christians being too little-known to write about or well-known enough to be blamed for a calamity: I see no contrast there. There's nothing at all unrealistic about a group that most people pay no attention to most of the time getting blamed for something, either without most people having any idea what the blamer is talking about or with people recognizing the name even though that name never had much attention called to it otherwise. For the latter, imagine if somebody famous were to try to pin some big bad thing on Quakers or Mennonites or Bahaʼi... or Mormons, outside of a few western states. For the former, consider any of dozens of cults centered on separate individual gurus scattered all around, which are active right now but you haven't heard of and won't until they do something newsworthy. Or, consider Obama & Clinton blaming the behavior of the mob at Benghazi on some video that did exist but which nobody had seen or heard of before. (It's not often admitted by people who don't buy into the conspiracy theories about what happened later, but one doesn't need to buy into those in order to admit the fact that the Obama Administration really did say that, or that it really was thoroughly obscure until they did.)
No in terms of religion 2) is the one that reflects most of recorded history.
How could a religion start without a real human figure as it's central supernatural deity (because Jesus was described as the supernatural scion of God)? Well the answer is that it probably started in exactly the same way as all the hundreds and thousands of other religions started, where the supernatural deity at the heart of the religion never existed at all (although for all of those religions the deity was claimed to be certainly real ... and lots of witnesses claimed to see all of those thousands of deities doing all sorts of things on the Earth and in the skies).
If people thousands of years ago could invent a fictitious god for all those other religions...
But the problem is that they didn't "invent" them. There are at least two massive differences between what normally happens with religious stories and "invention".
1. Gods, and iconic humans of a distant past like Adam or Abraham or Pandora or Achilles, are not suddenly made up all at once by one particular story-teller. Remember, people
believe(d) in them, and people don't just believe in a sudden new invention they've never heard of, but they do believe in, and pass on slight modifications of, ideas their cultures already had all along going back thousands of centuries. They evolve age after age, from an essence of nature or such, to gradually getting more & more personified, to getting some other personality trait mixed in that wouldn't have originally fit, to depersonifying back more toward a vague concept of an essence but a slightly different essence now, to shifting over with a new set of cultural paradigms to personify when something else about the tribe's culture or circumstances change, and so on, continuously, from one human lifespan to the next. The changes can be drastic but they take a long time. Nobody gets to just say somewhere along the line "I've just invented a new one, so now you'll all believe this one I just concocted is true" and be taken seriously by people who didn't have such a belief before. That's just not how cultural evolution works.
2. They also generally developed from the sky down, not from the ground up. They're results of the human tendency to either act as if parts of the natural world, like the sky or the sea or trees, thought like humans, or use a hypothetical single representative individual as a stand-in for psycho-social concepts like sin, generosity, agriculture/nomadism, and violence. Their role in a story is, or at least starts out as, to
be the sky itself or violence itself or whatever, just in human-like form, and as such, they're not expected to be anywhere near the same place & time as the audience or seem very much like real people the audience would have ever interacted with. Their names are often even simply the word for the thing they represent; Fortuna, for example, is not just the
goddess of luck/fate; it's also actually just fate, so when you say "Fortuna" does something to you, you're literally saying luck/fate does it. Enkidu didn't have horns/antlers because somebody just felt like making up a story about a guy with horns/antlers; he had them because pairing him with a king of a great civilization (Gilgamesh) was pairing the civilized and the wild together, so he was not just a guy but a "wild man" archetype, wildness itself (or at least an in-the-wilderness lifestyle) personified, just as a king was his civilization personified. That contrasts wildly with Jesus, who had no particular part of the grand conceptual order of the cosmos to be a bipedal form of.
So the question isn't just whether most religious (or otherwise culturally iconic) characters have been based on real people or not; it's what the alternative origin for them is instead of that, and what role it plays in its culture. And it's normally not just "somebody just suddenly made it up and others immediately fell for it", but something else more gradual, distant, gradual, non-human, symbolic, meta-cultural, and gradual, not just somebody suddenly popping up from nowhere and dropping a shiny brand-new myth on everybody else and talking them into starting a new religion based on it when the audience didn't have any such belief ten minutes before hearing this sudden new story.
Spontaneous invention all at once
ex nihilo that immediately actually convinces a bunch of people to seriously believe something they didn't before (not just retell the story for fun) seems to be unprecedented elsewhere. I don't know of a single established example. Where can I find one? Gods and cosmic-icon-type human characters don't count, because of both how long they take to develop and where they develop from. And if Jesus's origin isn't either of those two, then "embellished from one or a few real people" is all I have left, especially given that we have independent documentary corroboration of the presence of precisely the kind of people who would need to have been around for him to get embellished from.
