The central tenet of Christianity is that we should act as Jesus would.The respondent seems to be going along with Tillich's modernist interpretation: i.e. that the existenz/nonexistenz of a historical Jesus is a moot point because the heart of the Christian message is the moral example that we attribute to Jesus.
What we think Jesus would do is,at best, only loosely related to whether we think Jesus actually existed and did what he is said to have done.
The "truth-value" of the central tenet is T. We should act as Jesus would.
Fair enough. I see in this similarities to the Deist Christianity of, say, Thomas Jefferson - for whom Jesus was just a man with some pretty good ideas of how to act toward each other.
But my question, to play Devil's advocate, is this: if Jesus never existed, then from where does this moral example come? Who attributed these sayings and actions to a man who didn't exist, and why?
Furthermore, even if a historical Jesus did indeed exist (and I believe he probably did, although he was probably nothing like he appears in the Gospels), it is still a very difficult, if not impossible, task to determine what he would have done in any given circumstance. Like it or not, attempting to predict what a given person - fictional or not - would do in a given situation is intrinsically an interpretive, speculative exercise. Thanks to the free will that people are always touting, we can predict human responses with no certainty. And given the propensity of people to find justifications for their actions, there's a whole host of possibly-questionable things one can argue that Jesus would have endorsed, but that we will simply never know for certain.
Now, of course one will argue that this is one of the tasks of theology: to make educated guesses about moral choices given a limited set of holy precedents. And again I say fair enough. But what I would like to bring to your attention is the fact that these guesses must be dictated at least in part by the zeitgeist. Because the actual Jesus is apparently less important than the kinds of behavior people think he would be ok with - and because we can't exactly call him up and ask him - then ultimately the decisions of moral/immoral good/bad are in the hands of the people who claim to speak for him.
So far so good: that's organized religion for you. There's a caste of divine functionaries who are believed to have the ability to speak on behalf of the departed precedent-setter.
But (and this is coming full-circle now) if one claims that there needn't have been an actual Jesus, or that Jesus' divinity is not an important part of the equation, then those people who claim to speak on his authority therefore have no divine backing - seemingly by their own admission. Instead, we have a bunch of people arguing about whether a fictional character would approve of or engage in such-and-such a behavior.
So why, then, can't we cut out the middleman, and instead have these debates about whether such and such a thing is acceptable in the abstract? Why can't we have secular ethics without the distraction of a fictitious a desert mendicant peering over our shoulders? If Jesus' existence or divinity is not important, then one's choice of him as a role model is completely arbitrary (albeit culturally determined) - and the ethics attributed to him in any given era are the morals of that era, and not the other way around.