The existence of Jesus as a historical, living, breathing person is not seriously in doubt. There is a group of "Christ-mythers" who have bought into century-old theories based on incomplete data and faulty assumptions, but their contributions to the discussion may safely be ignored. What remains to be answered is who he was.
One popular expression of this issue is known as the Trilemma: "Jesus was either a liar, a lunatic, or the Lord". The assertion being made is that one of the following must be true:
Any good polemicist can tell you that is not a complete list of the possibilities. At a minimum, one must add the following to this list:
Let us examine each of these more closely.
The proponents of the "Liar" theory must find reasonable, consistent answers to the following questions:
It is plain to see that, while Jesus did experience a certain amount of popularity, it was with the "wrong sort". He did not appear to enjoy much of any material benefit from his lifestyle. For most of the three years of his ministry he was faced with an uncertain following and opposition from the religious and political elite. In the end, if he was just a liar, all he received for it was a shameful death. Anyone in his right mind could see that's not worth lying for.
So, was Jesus in His right mind? The "lunatic" theory seems to give a more reasonable answer to at least some of the questions posed. It certainly seems that Jesus must have fervently believed everything he said about himself and was willing to pay the ultimate price for those beliefs. Still, there are questions that must be answered, mostly related to his followers.
We know of many ancient and modern examples of insane persons with large groups of followers. Some built up their following while they were only "edgy", then did something big-time crazy which got them arrested or dead and then swiftly unpopular. Their names and their singularly heinous acts are remembered, but not fondly, except by a disturbed few. But, many of these leaders simply believed things that were obviously not true to begin with, or were later clearly shown to be not true. They were deluded, but not necessarily dangerous, and some still have a following. However, most sane people can easily recognize a delusional personality. Jesus' followers had ample opportunity to observe his behavior and test his claims against the surrounding reality. But it was not until the very end, when Jesus submitted himself to capture and trial, that the disciples fell away, mostly in fear for their own lives. Given Jesus' extravagant claims, it would seem they would have abandoned him much sooner if there were not ample evidence of the truth of those claims.
You may remember the story of "Miracle On 34th Street". A man claiming to be Santa Claus is forced into a competency hearing. His clever defense attorney asserts that his client must be found mentally competent if it can be shown that he really is who he says he is. First, he maneuvers the court into allowing that Santa Claus actually exists. Then, he shows that his client not only looks like Santa Claus, but also acts like Santa Claus in every way. Finally, he produces evidence that an authoritative body has pronounced his client to be the one, true Santa Claus. The court has little choice but to declare their agreement and Kris Kringle goes free.
Similarly, in order to show that Jesus was "reality-challenged", one must show that he had good reason to think that the Messiah did not or could not exist, and that he was not or could not be the Messiah. To reach the point quickly, given the existing evidence, he was being completely reasonable. Every circumstance of his life appears to point to the fulfillment of Messianic prophecy and the possession of divine power and authority. One can only reach an opposite conclusion by asserting that the reports of that life are not correct. Another way to state such an assertion is a variation on the original: "The New Testament writers were either liars, lunatics, or completely mistaken".
The "Jesus Seminar" group subscribes to just such a theory. According to them, the Gospels aren't anything like accurate eyewitness accounts. These folks have decided that Jesus didn't say much of what he is credited with saying, didn't perform any miracles, didn't make Messianic claims, and certainly didn't rise from the dead. The Four Gospeleers got it almost all wrong, they say. Actually, they don't even believe that the ascriptions to Matthew, Mark, Luke, and John are correct. What we have, they say, is a collection of legends that was passed down in an oral tradition, embellished and distorted for many years before being written down. Interestingly, they have no hard evidence for any of this. When this theory was first introduced, it was not completely unreasonable, given that the best date for any piece of the Gospels was well into the fourth century. What is remarkable is that these theories continue to be held by polemicists even though we now have texts that have been reliably dated to the early second century, and may date to the late first century.
There are many theories of Gospel formation, and I don't want to bore you with an exhaustive list. The important point is that exactly one of those theories is supported by actual physical manuscript evidence. That one "theory" is that the Gospels were written by the people to whom they are ascribed and that they are an accurate portrayal of the events and teachings of Jesus' life. Any other interpretation is completely speculative and requires a full-blown and wildly successful conspiracy by the early church in order to explain the uniformity of the existing manuscripts.
If the Gospel writers were liars, then they were amazingly unselfish ones, receiving exactly the same reward as their master: persecution, imprisonment, torture, and death. If they were lunatics, then they were remarkably consistent ones, producing a systematic theology that has stood for two thousand years despite early and strong attempts to destroy it. Of course, they still might have been mistaken. Even if every word of the New Testament is true, even if the writers described the events completely accurately from their point of view, even if they touched the scars in Jesus' hands, they could still be wrong in the conclusions they drew. Even if Jesus was neither a liar nor a lunatic, and every occurrence described in the Gospels is accurate, we still cannot definitely conclude that Jesus was who he said he was. Even if he fulfilled every prophecy, performed every miracle, made every claim, and spoke every word, you don't have to believe in him. Even if he rose from the dead, appeared to his followers, and ascended into the sky, he could still be mistaken.
Sadly, there is no counter-argument to this position. Anyone who accepts all the evidence, but not the obvious conclusion, is logically correct in doing so. You can quote the old saw of "quacks like a duck, swims like a duck", but in the end, it is still a matter of being convinced, rather than being proven. However, it does come down to that. The evidence that we've been given cannot be explained away as a fabrication or a delusion. These things happened. All that is left is to become convinced in our own minds about the meaning of those events. Either they mean that Jesus is the Messiah, the Son of God, or that, despite every outward indication, he isn't.