There are a number of misperceptions about reality here... not just the past, but the present as well. Let's start with the most obvious ones and work from there.
I really love this first sentence: ...eyewitness accounts written only 40 years after Jesus died.
You're kidding me right? This is the theists big evidence for the life of Jesus? I'm only in my mid-thrties, and I have a great memory. But ask me to recite the words of a speech I heard, even a great one, beyond a few key phrases or perhaps the gist of it... hell, can you even remember who your college English 101 prof was and you had them several times a week for 12-16 weeks? Even emotionally charged events where we might be expected to remember more details, like when a crime was committed, people make notoriously poor eyewitnesses at trials, even to contradicting the physical evidence. Our memories aren't that great in the short term, remembering the kinds of details suggested in the Bible, unless it is being recorded as one goes, is rather unlikely. Given that many of the characters in the Bible are illiterate, this doesn't seem particularly likely. And you want them to remember details for 40 years without altering them?
This does not even address the contradictory accounts that exist in different books of the Bible.
Secondly, I hate to point out the fact that the average lifespan in the time of Jesus was less than 40 years. That means that if it wasn't written down for 40 years, then it was at least secondhand, and probably more than this. There would also have been very few contemporaries of Jesus left alive, even those who had been children at the time.
Thirdly, it was common practice in literature of the day to embellish stories to make them more enjoyable. Writers would take some true facts, sprinkle them liberally with fiction, and market it as non-fiction. This is part of why classical literature requires several sources to establish what is and what is not true. Uncorroborated stories might as well be entirely made up for all we know. And since this kind of "elaboration" was standard practice, no one would have said anything even if they had known the that it was made up.
All this just goes to show just how truly unreliable the Bible is. It might be "true" in the sense it's what observers believed, or it might be completely fabricated. However, even if people of the believed it was true, they were also believers in Isis and Zeus. They were also likely to believe that David Copperfield had real magical powers and was capable of placing a curse on people. And these are the people this theist is trusting the veracity of the Bible to? Believing a thing and it being true are not the same thing. And whether Jesus believed he was the son of god or not, does not having any bearing on god's existence. Indeed, he's far more likely to have simply been a megalomaniac.