I've heard an answer to that question on several occasions. Typically, the reason people give me is, on the surface, based on a rational view of facts. Yet, when I dig a little closer I find that at all times an "unbelievers" view is first based on certain key suppositions.
Here's a few:
1) There is no God. If there is a God, the Bible hasn't accurately recorded who he is.
2) A book as old as the Bible, can't have possibly been accurately preserved.
3) The Bible is full of contradictions
These positions that people often take sound as if they are born out of a vast weight of evidence, but at their core is simply a "leap of faith". In other words, I've heard, "A book that old can't possibly be accurately preserved." That is a supposition. The person who holds it to be true figures it has to be true because it feels true. Often no evidence or rational thought will convince them otherwise.
Frankly, the verdict on the Bible has been in for years. There is far too much evidence on the reliability of the Bible to report here. Challenges such as Dan Brown's "DaVinci Code" won't topple the authority and veracity of scripture any more than a mouse could nock over the Washington Monument.
It comes down to this: people believe what they want to believe. If some one wants to believe that the Bible is to old to be properly preserved and translated, then no amount of evidence will dissuade them. If some one wants to believe there is no God, that is what they will believe. You might give them a convincing rational for the existence of God and the veracity of the Bible, but their rejoinder may still be, "But there couldn't possibly be. . . "
An individual who is willing to take the time and effort to fully investigate the claims of Jesus Christ and the message of the Bible, will come face to face with the truth: Jesus is God in the flesh. He entered into history and paid for human sin so that we would not have to pay the price ourselves. Accepting Christ as God and savior, opens the door to have a relationship with Him, and to eternal life with Him.
At that point, regardless of the proofs, you'll still have to decide if you are willing to believe. Saddly, many will face the truth of Christ and say, "I just can't believe it."
So much for intellectual honesty.
Adam