A common objection to Christianity goes something like this: Why do you base everything on faith? Why don’t you offer some proof? If God really does exist, then it should be far more obvious than it is. There should be conclusive evidence. Everyone should know it. If it were actually true, then faith would be necessary.
I believe that the problem with this objection comes from two mistakes. First, concerning faith. Second, concerning God. Let’s start with faith. Faith is a kind of trust. Now trust is required whenever something is hidden from us. For example, I believe that my wife is not cheating on me anytime I leave the house. Trust is required here because I cannot directly verify what she is doing when I am not around. Similarly, I trust that she really does love me and doesn’t secretly hate me inside her own mind. I cannot see directly into her own mind. So I have to have faith that when she says I love you she means it. So faith is required anytime something is hidden from me.
Now let us turn to God. In Karl Rahner’s phrase God is the Absolute Mystery meaning that God surpasses all conceptualization. A conceptualization is a classification of something into a kind or type. Since God is utterly unique, he is not classifiable into a higher kind or type. But if God is not conceptualized, then God cannot be reasoned about. All reasoning involves classification. Reasoning concerns placing individual objects into larger categories and relating those categories to one another. For example, Happy people eat oatmeal takes two classes or kinds of people (the happy ones and the oatmeal eaters) and claims that a certain relation exists between both groups (all those in the happy class are also in the oatmeal eating class.) Reasoning involves classification. So if God cannot be classified, then he cannot properly speaking be reasoned about. That means that we cannot directly prove that God exists. Proof is impossible because proof requires reasoning. Anything that can be proven (and so reasoned about) cannot be God.