I've got to say that I really respect Steven Den Beste because he is one of the most intellectually honest people that I have come across. He describes himself as a "mechanist atheist" who believes that the material universe is all that exists, and that it is wholly governed by the laws of physics. He has written a rather detailed essay explaining why his beliefs, and atheism in general, must be based on faith.
This is a point that most of the atheists I know refuse to conceed. I've argued endlessly merely trying to demonstrate to people that atheism cannot be rigorously proven to be true, and more often than not my logic is dismissed with handwaving and appeals to the "intellectual authority" of some writer or another. This dismissal of logic and reasoning only further demonstrate the religious fervor of some atheists and their refusal to accept the fact that their beliefs do, in the end, rely on faith.
Of course, there are some Christians who believe that God's existence can be proven; most of those also claim that it can be shown that the God who can be proven to exist is the Christian God and not the god of some other belief system. I think that these Christians are deluding themselves. If you claim to believe what the Bible says, then this is the only possible view:
Hebrews 11:1-2,6 -- Now faith is being sure of what we hope for and certain of what we do not see. This is what the ancients were commended for. ... And without faith it is impossible to please God, because anyone who comes to him must believe that he exists and that he rewards those who earnestly seek him.
Read the whole chapter, it's all about faith. Hebrews 11 describes the actions taken by dozens of people who put their faith in God, despite the fact that they could not see him or touch him. If God wanted to prove his existence he could certainly do it, but he has intentionally not done so because he wants us to come to him in faith.
Why? Well, that's a good question. I'm not entirely sure. Most likely it's a combination of two factors: his love for us, and the gift of free will. If he acted in such a way that his existence could be proven, then it would negate the usefulness of the free will that he gave us. I believe that when the Bible says that we are created "in his image" our possession of free will is a fundamental component of that. And because of his love for us, he didn't design us to be mindless robots. He wants us to love him in return, and love that is forced is not love at all.