Jesus was at least a human being, male, who lived in Palestine in the first third of the first century. Human Jesus is not the Jesus of faith. Christianity is a post-Easter religion; its subject is Christ after death. But with David Friedrich Strauss in 1836, historians began the Quest for the Historical Jesus.
No physical artifacts exist attesting even to Christianity until 160. Only three non-Christian documents mention Jesus within 100 years of his death: a brief note by Pliny the Younger; a mention in Tacitus’ Annals in connection with Nero; and two passages in the Jewish historian Josephus’ Antiquities that say Jesus was a wise man said to work wonders and was crucified.
Everything else comes from believers. Sources outside the New Testament, including infancy gospels, were written 100-200 years after Jesus’ death and are fictional. So the only non-trivial record of Jesus’ biography is the New Testament. Data outside the four Gospels include nine bare facts in the letters of Paul, including that Jesus was a Jew, had brothers, 12 disciples, a last supper, was betrayed and crucified.
Gospels are problematic sources. Unless you’re Evangelical, contradictions are clear. (Look at the infancy stories of Matthew vs. Luke; the death scenes of Mark vs. John.) 200 years of debates center on how and what to trust as fact. Scholarly consensus: Mark came first, about 60-70AD; Matthew and Luke used Mark a decade or so later; John was last and independent. None knew human Jesus.
Criteria applied include favoring data that are earlier, appear in multiple sources, and are less likely to be made up (e.g., crucifixion, considered humiliating). The authors were educated Jews with the Hebrew Bible top of mind; Matthew, in particular, includes many details meant to mirror David and the Prophets. The “prophecies” of Jesus found in Isaiah and Psalms could easily be retrofits.
Using criteria, what does human Jesus look like? Definitely a Jewish male, born around 4BC in Palestine under Roman occupation. Also, most likely:
He was from a small semi-rural town of Nazareth in Galilee and spoke Aramaic. Had brothers, including one named James, and perhaps sisters. His parents were named Mary and Joseph; dad worked with his hands, and Jesus learned such a trade before abandoning his family at about age 30 to follow an apocalyptic guy named John the Baptizer.
Jesus himself held apocalyptic beliefs – “the time has been fulfilled, the kingdom of God is at hand, repent and believe in the Gospel” (Mark 1:15). He knew the world would end soon, God would rule a place on Earth where mighty would be humbled and humble glorified – an inverted world. This would happen soon (“Some of you standing here today will not taste death until you see the kingdom” Matt. 6:28). Alarmed, Jesus collected 12 male followers to parallel the 12 tribes of Israel.
He preached itinerantly for a year and ran into minor trouble with some strict Jews, but was observant. Was widely rejected. Did not believe he was God – but a kind of prophet. Told parables. Counseled keeping the Commandments, giving up possessions, loving others but leaving your family: time was short. Hated the rich. Associated with social misfits, criminals, women. Reputed to be a healer of the sick.
Took the 12 to Jerusalem at Passover where he offended Jewish authorities, perhaps because he predicted the Temple’s destruction; they complained to the Romans. Jesus had a last meal with his Disciples. He was betrayed by Judas, who may have revealed Jesus called himself King of the Jews (in the new post-apocalyptic Kingdom). Romans arrested him; a brief trial; executed by crucifixion that day, abandoned by his friends.