You don't have to convince millions of people all at once, in one generation, that they saw something, or even that their parents did.
You could, instead, convince a little cult today, and get that cult to grow over several generations until it is millions strong.
Or, instead, you could convince a small priestly caste that thus-and-so is a good legend to write down in what becomes the holy book; and teach millions of followers that everything written down in the priests' holy book (which they can't read) is true. Then some generations later, when there is an established tradition that whatever the priests' book says, is true, when the masses finally do get to read the priests' book, they will believe it too.
This is all obvious to me. It was pointed out in detail (along with other possibilities) during the debate, by myself and others. It was considered "unpersuasive."
I actually made headway with one person (honestly he's one of the most reasonable, fun to debate with guys I know) by saying "look, even if you consider the (fake story gets propogated somehow) idea impossible, divine miracles are ALSO impossible. So until we find more evidence, Mass Revelation can't possibly be MORE likely than 50%.
Something I've been hearing a lot lately (specifically from Orthodox Jews, although it comes up a lot in debates about religion) is that having a large number of people telling a story makes it more likely the story is true, because multiple witnesses can call each other out for deviating from the truth.
My gut reaction is that this is extremely false. But it's a point that should be scientifically testable, and I figure that someone should have done a study on it by now. Does anyone know of such a thing?
A related issue is the argument that oral tradition meant something very different thousands of years ago, when it was the ONLY form of historical record. Oral historians were duty-bound to preserve the story. This sounds plausible. It probably ISN'T as easily testable since we can't compare oral history from pre-writing times against... well, much of anything. (Well, I guess archaeological evidence, if the events being described would have left enough archaeological evidence). Is there an official, accepted scholarly opinion on this?