Many arguments against Christianity are based on the fact that we must have faith without reason. This has spawned a whole new religion/philosophy developed by Ayn Rand called Objectivism. Objectivism is based on the principle that reason is man's only means of knowledge, and that everything we know is a result of our sensory perception of the world combined with the laws of logic. I see so many atheists trying to refute the existence of God because it does not follow the rules of logic. The existence of the Bible, and thus God, cannot be proven within the limits of pure reason, or reason as we know it. To prove that God exists, would require an additional *divine* truth, that is reason that is not contingent upon the laws of nature. In other words, it is impossible to assert the validity of the Bible on reason, as we know it, alone. This means to have total truth, we must understand this missing piece of divine truth. And this missing piece of divine truth can only be understood from a relationship with God. That is to say that our feeble minds cannot express this missing divine truth in mere words; it is above our intellect -- it requires transcending our earthly existence to understand something greater.
So why would God leave out this missing key of divine truth? God's motive in creating our existence is to develop souls who would love Him unconditionally. Love is greatest when it is free love. If God were to allow His existence to be totally proven, how could we have free will? All of our actions would be based on the fact that we knew that God exists. A crude analogy could be a famous entertainer comes over from a foreign country, where he is very well known, to a small town in the US, where no one knows of his fame. Now if this famous person wanted to make friends that would truly love him/her for the person they were and not the fact that she is some famous star, they would have to withhold a vital piece of information about themselves, or their decisions will be biased, good or bad, by her fame.