Let me just say I would LOVE to jump into this debate, but it's late and I realize I don't have the time necessary to spend on it. So I'll just throw in some general remarks for a start...
I have debated the historicity of Jesus with all sorts of conspiracy theorists off and on for the past three years or so, on IMDB.com (boards like "The God Who Wasn't There," "Religulous," and "The Da Vinci Code" amongst others). I've sometimes argued by myself, sometimes with other religious folks and sometimes had atheists on my side. One thing to keep in mind is that no matter how much you might disagree with Jesus' reputed teachings or the practices of Christians or their theological beliefs (including belief in scriptural inerrancy), this is irrelevant to whether the man actually lived.
Note that I use the term "Myther" here not in a pejorative sense, but to refer to people who think Jesus was just a fictional character or hoax, rather than a real human being.
Now this is one of those debates where people are quick to accuse me and say "well, as a Christian, you're biased." But if I were shown overwhelming evidence that Jesus never existed as a historical person, that would pretty much shatter my faith. And I wouldn't want to believe in a system I knew
was nonsense. For me, the foundation of Christianity is that Jesus was an actual person. If he was merely some metaphor, or a plagiarism of some other gods or unrelated persons, that's it, as far as I'm concerned. Time to consider some other religion (Conservative Judaism strikes me as a pretty good one)!
For me, it's not merely a matter of an appeal to authority (ie: that the majority of modern experts in the related fields, whether believers or non-believers accept that Yeshua bar Yosef, known to us as Jesus of Nazareth, called Christ, was a real historical person, upon which the religion known as Christianity, and the character of the New Testament, was based), though this helps (the burden of proof is on the minority challenging the experts, here). Rather, as a budding "expert" myself (though by no means at the top of the game yet, I assure you) the evidence I've seen thus far is convincing. There's multiple attestations of a figure who sounds an awful lot like Jesus, a Jewish cult leader with a brother named James, in the right place, and the right time... both in non-Christian and Christian sources (the latter, though biased, are useful, especially since several of them are independent of one another). Some are quick to point out that these scholars don't all agree exactly what Jesus was really like, or precisely the main focus of his teachings. However that is a bit like saying because mainstream scientists disagree over the precise mechanisms or systems of evolution, that evolution is "in crisis" or "just a theory" (and somehow alternative theories like creationism are just as valid). The point is they agree on certain minimum things, sufficient to say he existed.
Then there's the absence of any critiques of Christianity charging that Jesus was a hoax (which one might expect if that's how it happened, or at least a forceful defense by a Christian apologist early on that he wasn't). Finally, the fact that the earliest attestations are from contemporaries, within a very short time frame (for antiquity), basically a few decades after his death, makes it less likely that he was simply invented.
The notion that all the texts were tampered with later by Christian scribes needs to be demonstrated. The only reference we know is tampered with is the infamous "Testimonium Flavium." Even still, most scholars believe that it is still partially genuine. The references to James elsewhere in Josephus are not contested by modern scholars, and in context there is nothing that appears out of place for a Jewish commentator like Josephus (who is discussing the fall of a high priest connected to the death of James).
The arguments that earlier gods were identical or very close to Jesus is probably the weakest Myther argument, and is at the bottom of the list of acceptable evidences for Mythicism, but I generally ask "Copycat Mythicists" to show from pre-Christian texts the evidence for these other gods having "Jesus-like" characteristics. Typically, all they can do is cite vague "lists" that have appeared in non-scholarly works since the 19th century.
The idea that if one strips away the "supernatural elements" (the theological claims about Jesus which history alone cannot prove or disprove), one ends up with a "different Jesus" than the Jesus of the bible, and so therefore somehow indirectly proves Jesus was a Myth, is, I think a poor argument.
That would be akin to arguing that Sathya Sai Baba is a Myth, because his followers believe him to be a Godman, possessing divine attributes, and able to do miracles, possessing wisdom beyond that of mere mortals, while the rest of the world tends to regard him as some kind of deluded street magician, if not an outright con. The "Mythic" argument is that Jesus never existed, not that some of his powers were exaggerated, or some of his teachings untrue. That would be shifting the goalposts. Agnostics like Bart Ehrman have no trouble doubting that Jesus was divine, or had any kind of supernatural abilities, yet nevertheless accept his historical existence based on the evidence which explains the origins of Christianity better than some kind of convoluted invention conspiracy.
And finally, admitting that someone "just like Jesus" at the same time as Jesus, existed, is conceding the argument. Saying "well it was a common name back then" is a non-argument (but I hear that one often).
I'll check back in awhile. If nothing ever comes of it, on the one hand I'll be disappointed I didn't get to have a hearty debate with you all like in the old days, but on the other I'll be pleased that another internet conspiracy theory has failed to gain a foothold in these hallowed forums...