To answer that question, I must first digress to explore how we know anything. All of our so-called "knowledge" involves an act of faith. I trust what my senses tell me, at the basic information-gathering level and at higher interpretive levels as well. Although my closeted brain has assembled an inner image of each person I have met, I realize that the image involves a large measure of trust. I trust that Tom is not wearing a costume, and that he indeed works for UPS and is not a burglar scouting my house.
I think I know him, but how can I be sure: Perhaps Tom is an identical twin who job shares with his brother. So many times people have surprised and misled me. I have learned that one of my best friends had a secret life of sexual addiction, that another was abused by her father for 15 years. I thought I "knew" these friends, only to find I was missing vital information about them. All human relationships involve a kind of approximation. It preserves "otherness." We always fall short in knowing another.
Nevertheless, at the most basic level I trust that these friends actually exist as individual persons much like myself. How can I know for sure? The problem of "other minds" poses a major puzzle that has exercised philosophers for many years. I know that I exist, and I think I know my own mind. But how do I know your mind? I believe, by analogy, that when you shut a car door on your finger, something inside your brain happens that closely resembles what I experience when I slam a car door on my own finger. Yet I can never know for sure, because I can't get inside your mind. I must take your word for it when you tell me how much it hurts.
How do you know that I exist? Perhaps Philip Yancey is a pseudonym. Perhaps this is being written by a committee, or by a programmer at Fuller Seminary who cleverly devised software to crank out popular theology. If you try to contact me on the Internet, you will never know for sure whether it is I responding, or simply a concocted screen name. To me, I am an I; to you, I am a you, and that distinction introduces a powerful strain of uncertainty.
Admittedly, most people don't go around questioning whether other minds and persons exist. We take it for granted, without giving it much thought. Yet as we encounter other people, every one of us assembles a different composite of that person. Think of the authors of the four Gospels: Matthew, Mark, Luke, and John were each struck by different aspects of Jesus' personality and life. As they reflected on what they knew about him, different words and scenes came to mind. Or consider the disciples: 12 of them followed Jesus around for three years, but what different conclusions Judas and Peter drew about him! Later, a Pharisee named Saul of Tarsus thought he had Jesus figured out, until a personal encounter radically changed his opinion, and altered the direction of his life.
If it's difficult to know other people, how can we know God?
Perhaps what I have learned about the process of knowing other people can shed light on questions about knowing God.
First, I recognize that knowing other minds involves an act of faith, whether I am speaking of other persons or of God. We know other minds indirectly, not directly. I assume you exist, and have an outlook on the world somewhat like mine, but I can never be certain. Alvin Plantinga applied this problem to the question of the existence of God. I cannot be certain of God's existence, he acknowledges; I cannot prove it. Yet I can't be certain of anyone else's existence either. I believe I am not alone in the universe, but because I cannot perceive any other person's mental state, I must accept this belief by analogy—or by faith. In fact, I have as much evidence for believing in God as I do for believing in other people. Although both lie beyond proof, I accept both as a basic, foundational assumption.