Historians also agree that the biography of Jesus that modern Christians hold as true was an amalgamation of literally hundreds of radically different, incredibly diverse biographies of Jesus written by hundreds of different authors, almost all claiming to an eye-witness account by an apostle. The council of Nicea met in 325AD and decided which parts of which testimonies were gospel and declared the other 99% of testimony heresy. Historians agree that Constantine's motives were not particularly Christian or even religious but polticial, choosing the favorite version of the rich and powerful, most particularly the favorite versions of his mother, Helena. No single individual had more influence about the story of Jesus as told in the Bible than St. Helena and we owe much of our modern ideas of a virgin mother, the trinity, the transmutation of blood to wine, etc to that women's particular prejudices. After 325AD, most of the hundreds of eye-witness testimonies of Jesus were hunted down and destroyed to make certain the Roman Emperor's version of Jesus was the version that you believe in.
The most contemporary Jewish historian, Josephus, writing 60 to 80 years later, had little information about who Jesus was or what he believed and certainly did not seem to have ever heard that he was supposed have risen from the dead Josephus does write of Paul and Peter and Jesus' younger brother James and how the violence between them contributed to the overall violence that ended with Masada, the destruction of the Second Temple and the beginning of the end of Judaism in ancient Palestine. The main issue between them was that Paul had invented something called Christianity and believed that the Sermon on the Mount was a message to be shared across the Roman Empire with non-Jews and foreigners and even women. James apparently believed that his brother Jesus was first and formost a Jew and a Rabbi and a Prophet and that to follow Jesus you firstly had to be a Jew in good standing.
I think an intelligent reading of the New Testament reveals that there was more than one man preaching the forgiveness of a loving god and the promise of eternal life in and around Palestine during the reign of Augustus Caesar. John the Baptist was an early, super popular version of this figure and his execution by Herod served as a kind of model. The Jesus who came from Egypt is almost certainly different from the Jesus from Bethlehem and the Jesus from Nazareth. Jesus the Rabbi is almost certainly a different historical man than Jesus the Carpenter and Jesus the Fisherman and Jesus the Zoroastian wizard. Which of these said what or performed which acts, which of these were executed by Jewish Councils or Roman Governors is unknowable but these governments were putting a lot of rebels and preachers to death in this time.
Christianity does not seem to be the message or intent of Jesus, who barely ever spoke of founding some new religion, but of Paul, an eloquent charasmatic philosopher at the center of Roman Greek and Jewish society who understood what sort of Church and belief system was wanted and would satisfy the popular imagination. Chrisianity should be Paulism because Paul invented the religion, not Jesus.
Rather than ask why the apostles behaved as they did if there were no Christ, this sober historian asks why anybody believes the narrative of those figures told by a
Roman Emperor more than three centuries later, particular when that Emperor had so little faith in the truth of his narrative that he felt the need to supress and
destroy all of the hundreds of alternative accounts that bore witness to a remarkable change in the philophy of the Near East, even while disagreeing utterly about the biographical facts surrounding the central figures of that change.