The question is not whether Jesus existed or not. It's obvious he was a real historical figure. No historian really denies this.
Sure, as long as the ones who do deny (or question) this are disqualified ipso facto as historians in your eyes...
The question is: how much that is written about him in the NT is historical and how much is embellished.
A subsidiary question would be, how does the "quantity" ("how much") of historical accuracy relate to the "either/or" issue of character identity? Iow, under what "floor level" of historical accuracy (or verisimilitude) do we stop calling that an "embellished" legend about one historical character and start calling it a fiction (although possibly inspired by a number of historical as well as literary characters)? This is a semantic question for sure, but a very important one imo. So many sensational "lives of Jesus" end up with a Jesus who doesn't look like Jesus at all... kind of "I met that guy, but it was not him".
The position that the gospels are firmly in the genre of fictional literature is, of course, the only possible explanation for atheists.
Certainly not. Thirty years ago or so the fashionable Marxist readings of the Gospels, for instance, would take the core of them (not the miracles of course) as historical, not fictional. And what about the atheists among the "historians" who do believe in a historical Jesus?
At the end of the day we cannot be certain about what is historical and what is fictional in the NT. But it is a mistake to insist on 100% certainty. If that were the test then we would have to reject all of ancient history. And it simply isn't good enough to reject the gospels simply because they contain miracles when there is a lot of evidence that supports their historicity notwithstanding the miracles.
This somewhat muddies the issue: uncertainty actually comes in at different levels, at least as to (1) what in the Gospels is deemed historically plausible; (2) which version of the historically plausible (Mark's, Matthew's, Luke's or John's or yet another) is to be retained as potentially historical; (3) how much of it is actually to be ascribed to the historical Jesus.
slim:
There remains, at least, a difference in literary genre.
Aatw:
You might be interested in Robert H. Eisenman's provocative theories, especially about James and early Jewish "Christianity" -- that may have been much less "Christian" than the extant "Jewish-Christian" works, which all presuppose the conflict with Paulinism, suggest. The open question is what role, if any, did the figure of Jesus play in those Jewish circles prior to the encounter with Hellenistic, and especially Pauline, Christianity?