I'm in the small but growing minority of Christians who don't actually think that the Bible is the primary means of understanding God.  In other words, I think that regular people not only wrote the Bible (and that it is an account of their experiences), but also that regular people have translated it throughout the ages.  Translating and interpreting the Bible is tricky even today, so I can't imagine how difficult it was in the Middle Ages or before. 

That's not to say that  I don't think the Bible is important.  I think it is--it's a fascinating book, but I think people often take it the wrong way.  I don't read it as a manifesto or "instruction manual from God", I simply read it as a collection of stories and experiences concerning the Judeo-Christian God. 

But for people who do take the Bible literally and think that it must be 100% internally and externally affirmative, the answer I usually get is "You just have to have faith that God has the power to preserve his word through time, at least enough to get the basic message of salvation across."  I disagree with that, though--my personal salvation experience had nothing to do with the Bible per se, in that I wasn't reading the Romans road or the Gospels and thinking to myself, "Hey, I'd better do this."  It was an extraordinarily personal experience--the singular experience which prevents me from leaving Christianity. 

That's probably not the answer you were looking for.