It's a question of faith
I loaned The Da Vinci Code to a friend of mine. That's how it all started.
He soon moved on to Dan Brown's Angels & Demons. Which led to some research into the Illuminati, then secret societies in general, to the Freemasons, culminating in The Hiram Key.
Now, for those who've never read The Hiram Key, it links ancient religions with secrets about the "true" life of Christ to the Knights Templar and finally to Freemason ritual. It's theory and conjecture written as fact and certainty. It's perfect for anyone who relishes the good conspiracy theory (and who knows? it could even be true).
He's now convinced that his religion, Christianity, is a sham. That the Bible got the history all wrong, therefore the entire establishment is false.
I just finished reading Saramago's The Gospel According to Jesus Christ and was eager for just this sort of debate.
But do the facts of what occurred in a 35 year span, thousands of years ago, REALLY matter? Christianity has been around for over 2000 years. Isn't the religion better defined by how the scripture is interpreted by its followers now? By the actions of the Church now? By the faith of its followers?
Am I the exception in feeling that the absolute infallibility of the "historical facts" have nothing or little to do with faith? Does it matter whether or not Jesus was an only child or had a brother or a twin or a dozen brothers and sisters? Does it matter whether or not he was married? Does it matter whether or not he had children?
Isn't the point to learn the scripture and the lessons of the parables and strive to be the best person you can be? And to follow the laws of the Church (regardless of the historical accuracy of the beginning)?