Is there anything a philosopher can know for certain? For me, there's only one proposition that comes to mind, and that is that neither philosophy, science, nor any of our other arts will ever give us irrefutable answers to our basic questions about eternity and the meaning of life. No philosophical statement could be better supported by inductive evidence than this--there are questions about which reasonable people will always disagree.
For some, that's all you need to know about philosophy. It goes nowhere. For others, it doesn't matter. The questions are what's important, not the answers. But if this is the only proposition we can be sure of, what are its implications? What if we looked at it not as a conversation killer, nor as an irrelevancy, but as a starting point? If it is the only thing we know for sure, maybe it's important.
An obvious inference is that if there is a God, in the usual sense of the term, he designed the universe in such way that we can't prove his existence with certainty. Christianity teaches that he came to earth in human form and worked enough miracles to fill a library, so to speak. Regardless of whether the tale is true, it's clearly not enough. Even if it is true, reasonable people still disagree.
What if he stayed on earth and worked miracles all the time, and anyone could see a miracle simply by taking time to visit him? What if, for example, we could all go to see Jesus tonight and get him to do the water into wine trick? Would all reasonable people come away with the same opinion? Or would some come away wondering whether there could be some scientific explanation? To take it a step further, what if miracles were so common that no one could doubt them? In that case, scientific inquiry as we know it wouldn't exist. In fact, neither would the concept of miracles. We would all see that the universe had a certain unpredictability and irrationality to it, but would that necessarily imply the existence of a God? Not at all.
On the other hand, if Christianity and many other religions have it right, God is perceptible to those in the spirit world. If he placed us outside that world for a while, he did so intentionally. We arrive at three conclusions, then. First, if God exists, he designed us in such a way that we can't perceive him directly. Second, he designed the universe in such a way that it's impossible for us to prove his existence. Third, and most interesting, he designed the universe in such a way that it's impossible for him to prove his existence to us. And a fourth conclusion follows from the third--if there is a God and you are interested in knowing him, you can't expect proof.
So the question is what this means. How does it affect our understand of God? Plants and animals don't have to think about any of this stuff. Spiritual beings like angels are supposedly in the know. Why are we the only ones with all the questions and none of the answers?
Louise Glück's book of poetry The Wild Iris answers the question in a beautiful way, though not a way that's very satisfying to those who put their hopes in eternal life. "Grief is distributed between you, among all your kind, for me to know you, as deep blue marks the wild scilla, white the wood violet." Christianity answers it somewhat differently by saying it's a test of faith. The two approaches differ as to whether there's anything for the individual to obtain after she's served her earthly purpose, but they agree on one point--suffering and doubt aren't just obstacles to the fulfillment of our purpose. They're actually part of the purpose in some way. As one philosopher said, we are the eyes through which the universe is able to perceive its own glory. If so, it appears that part of what the universe feels is a sense of doubt and a sense of loss. But maybe also a sense of hope.