Even if there was an actual person who served as the model for the Biblical Jesus, it's unbelievable that the depiction in the Gospels is at all representative of him. Even assuming there were no conscious efforts to clean things up and make him larger than life (which is itself not really believable), not a single one of the Gospels was written until long after his presumed death. Human memory is not static; it changes as we use it. So even if the actual writers of the Gospels were able to talk to the disciples, there's no real chance that they (generally poor and uneducated men in awe of their teacher) could have kept things from getting skewed and slanted in their memories. Not even trained observers can manage that over a long period of time, when relating things that happened years or decades in the past.
Most likely, though, early Christians made lots of changes to the scriptures in order to answer objections from nonbelievers. And to bring the various stories more in line with each other, so that the Gospels were more consistent (thus to provide less fodder for criticism). And, of course, they had to try to make Jesus fit the numerous prophecies in the Old Testament (which, oddly enough, the Jews didn't accept). And they had to show that Jesus was divine in nature, rather than just being a man, so they could show that their belief was superior to other religious beliefs (such as Greco-Roman ones, which had lots of divine/semi-divine actors).
The more a person actually examines the Bible, and examines the history of Christianity (especially the early history, before the Council of Nicea decided what was to be Christian scripture and what wasn't), the less believable it all becomes. But that's really the point. The Bible isn't there to make a factual, intellectual case for Christianity, it's there to provide lots of stirring, inspiring stories so that people will simply accept it, rather than examining it. Just like most holy books and mythological stories, really.
