How do you know that anything from the New Testament even happened and that someone didn't just make it all up? There are no written records of Christ's existence, except the Bible. Even if there were, it would only show that he was born and died. His supposed claims and miracles are only recorded in the scriptures. Many scholars believe those texts were written several hundred years after Christ supposedly lived. What prevents the writers from simply having fabricated the story? It would be easy to correctly prophesize(sp?) "future" events in the bible if the whole thing is fiction and that wouldn't prove anything, except that the writers of the Bible knew how to accomplish the literary device known as foreshadowing.
Those are great questions. Remember that most of the books that comprise the New Testament were individual writings that were collected later on by the Church.
There are several other writings and historical accounts by others at the time. The Jewish historian Josephus is one. He wrote about Jesus in "the Antiquities of the Jews."
Now around this time lived Jesus, a wise man. For he was a worker of amazing deeds and was a teacher of people who gladly accept the truth. He won over both many Jews and many Greeks. Pilate, when he heard him accused by the leading men among us, condemned him to the cross, (but) those who had first loved him did not cease (doing so). To this day the tribe of Christians named after him has not disappeared
Phlegon was one of several Greek historians who wrote about Jesus. None of them in flattering way.
Now Phlegon, in the thirteenth or fourteenth book, I think, of his Chronicles, not only ascribed to Jesus a knowledge of future events . . . but also testified that the result corresponded to His predictions. (Origen Against Celsus, Book 2, Chapter 14)
And with regard to the eclipse in the time of Tiberius Caesar, in whose reign Jesus appears to have been crucified, and the great earthquakes which then took place ... (Origen Against Celsus, Book 2, Chapter 33)
Jesus, while alive, was of no assistance to himself, but that he arose after death, and exhibited the marks of his punishment, and showed how his hands had been pierced by nails. (Origen Against Celsus, Book 2, Chapter 59)
Perhaps the most revealing evidence is the fact that 10 of Jesus's disciples were martyred for preaching the gospel. Several were crucified. Would you go to your death by stoning or crucifixion for a lie?