This is an excellent question. Just because a book has been handed down through time accurately from how it was originally written, this doesn't make the preserved writing true! It could be fiction!
A way for us to have certainty that the Bible's claims about Jesus are true is to have sources outside of Christianity from the same time period which confirm the Biblical accounts. Often times, names of certain external writers are thrown around, but sometimes the exact quotes from these authors are not listed or referenced. I'll try to help by finding, listing, citing, and, or quoting such authors and what they said which confirms, not denies, the Biblical accounts concerning Jesus.
First, I'll start with Josephus, who was a Jewish historian. From Antiquities Book 18 Chapter 3, Josephus writes concerning Jesus:
Now there was about this time Jesus, a wise man, if it be lawful to call him a man; for he was a doer of wonderful works, a teacher of such men as receive the truth with pleasure. He drew over to him both many of the Jews and many of the Gentiles. He was [the] Christ. And when Pilate, at the suggestion of the principal men amongst us, had condemned him to the cross, those that loved him at the first did not forsake him; for he appeared to them alive again the third day; as the divine prophets had foretold these and ten thousand other wonderful things concerning him. And the tribe of Christians, so named from him, are not extinct at this day.
Note: Josephus was not a Christian, but he wrote this concerning Jesus, and Josephus was a historian! What he wrote is an excellent summary of Christ's life which fits with the Biblical account of Christ's life found from outside the Bible from a non-Christian historian. Antiquities from which this quote is taken was completed by before the end of the first century!
For more quotes from Josephus: http://www.josephus.org/quotes.htm