The general ‘undeniables’ are that a man named Jesus existed, was baptized by John the Baptist, and was crucified. It’s presumably safe to say that he was preaching something that was upsetting to the local Jewish and/or Roman authorities, given his crucifixion. While we’ll obviously never know precisely what he was saying, it is important to remember that the New Testament is a collection of independent documents, and so you can extract some common themes from them. He was clearly a reformer of some kind, something of a hippy, and appears to have emphasized some level of personal humility and charity. It should also be kept in mind that this era was a time of massive religious turmoil in the region, and Jesus was far from being the only radical reformer going around and preaching. He just happened to become very popular after dying.

While he obviously wasn’t a magician and didn’t come back from the dead, I do think there are certainly some themes in his message that are positive, and it’s not a coincidence that you see them echoed in other independent religious traditions. Of course, most Christians don’t live up to those values to any meaningful degree at all, but that’s another matter.