We first meet Jesus in the letters of Paul, all written in the 50s CE. Paul says he never met an historical Jesus, only a visionary one. He says Jesus pre-existed in heaven, created the material universe (which in gnosticism, God, being pure remote spirit, would never do, casting Jesus as the gnostic demiurge) and mediates between God and man. As for his earthly bio, he was born of unspecified parents of the line of David, may or may not have had a brother James, had a ministry to the Jews teaching the end-times, had followers including one named Peter, initiated the Eucharist (a Greek idea), was 'handed over' to 'the rulers of the age' who crucified him for unstated reasons, a process which involved the Jewish leadership, and was physically buried. That's it.
Next is Mark. He's writing in the mid-70s, after the destruction of Jerusalem (70 CE). His Jesus is an ordinary Jew born without prophecies or annunciations, who doesn't become the son of God until John the Baptist baptizes him (washes him clean of sin). At that point the heavens open and God declares that he adopts Jesus as his son (on the model of God adopting David as his son in Psalm 2:7, a point made explicit in Acts 13:33). What follows is the only (purported) account of Jesus' earthly mission, those in Matthew, Luke and, more remotely, John, being based on it. Why might it not be an accurate bio? Because its episodes can be mapped onto the Tanakh, passages which the author likes to think of as messianic prophecy, through which he moves his principal character as he wishes. This suggests that at the least the author was not concerned with history but with story, and that he knew little or nothing about his principal character at all ─ perhaps excepting some sayings attributed to Jesus (though as Crossan shows, which if any were from the one original source can't be determined with any confidence), and perhaps excepting some anecdotes, perhaps the most curious being that Jesus with just the one exception, never mentions his mother except in scathing terms (Mark 3:31
, Mark 6:3
, Mark 15:40
, Matthew 10:35
, Luke 11:27
. John 2:3
, contrast John 19:26
Was there an historical Jesus? Perhaps, but if so we know very little about him. And the argument is there that no historical Jesus is necessary to account for the documents of the NT, since the author of Mark has devised a bio independently of history.