This is a relatively short chapter with three main points. First, we cannot currently know God unless he reveals himself to us (149). Several proof texts are mentioned, including Matthew 11:27, "No one knows the Son except the Father, and non one knows the Father except the Son and any one to whom the Son chooses to reveal him" (RSV, italics his). For Grudem, Scripture must filter any other way of knowing God as well, including natural revelation. "The Bible alone tells us how to understand the testimony about God from nature" (italics his).
His second and third points respectively are then that 2) we cannot never fully understand God but 3) we can know God truly. God is infinite and we are finite. Therefore, we can never fully understand God. This is how Grudem defines the word "incomprehensible," not that we cannot understand God but that we cannot understand him fully.
This is true not only about God in general, but also that "we can never fully understand any single thing about God" (150). And it is not only true now but also in the age to come. Our inability to understand God fully is not simply a matter of our sinfulness but of our finitude, the fact that we cannot comprehend the infinite. "There will always be more to learn" (151).
It is important nonetheless for Grudem to qualify that we can still know God truly. "All that Scripture tells us about God is true" (151). Further, more important than facts is the fact that we want to know God as a person (152). Christians have the far greater privilege of knowing God personally above mere knowing facts about God.
Much in this chapter is commendable. It seems beyond question that we cannot know God fully or completely because he is infinite and we are finite. Surely Grudem is also right that it is more important to know God relationally than to know God cognitively. Finally, it seems possible that there are some facts we can know about God that are not skewed in any way. For example, "God knows everything there is to know about the creation" seems like a true statement that is absolutely true and completely literal.
To say something is literal is to say that we are using words in their ordinary sense. If I say, "he went through the roof," I am not speaking literally. To go through the roof literally is no doubt to die of massive head injuries from your body being propelled through all the wood and roofing materials. If I say "I have a son" and am talking about my dog, I am speaking figuratively. If I say, "I have a son" and am talking about my son, I am speaking literally.
It seems quite likely that much of our knowledge of God is figurative to some extent, not completely literal. To say much of what we believe about God is figurative is to say that we are saying true things about God but we are describing him by comparing him to something in our world that we can relate to. "God is my fortress," not because he is a literal building structure I can hide in when someone is trying to attack me, but because he is like that.
So when the Christians of the 300s and 400s wanted to describe the three members of the Trinity, they described them as "persons." I accept this language as true. Is it literal, or is it God helping the early Christians find something in our world that expressed what the members of the Trinity are like? The principle of revelation, one that Grudem fails to appreciate, is that God primarily speaks to humanity in terms we can understand. By contrast, Grudem seems to think that God reveals himself to us in his own absolute categories, that he lifts us up to his level more than he reaches down to ours.
Thus we return to the underlying problem with his hermeneutic, that he implicitly reads the Bible as a document that stands outside of human context. He treats the words as propositional statements of absolute truth rather than as words in genres written in situations in cultures, as God speaking to humans at particular times and places largely within their own categories and "language games." The meanings of the Bible were comprehensible to those to whom its books were first revealed; therefore, its original categories were largely those of the audiences to whom they were first revealed.
The Bible primarily gives us pictures of God, snapshots at particular times and places. Most of what we know of God we thus know more by analogy than on a fully literal level. But yes, surely we can know some things about God literally as well, without saying he is "like" something we can relate to. And Grudem is surely very right to say we can know God personally and relationally--by far the most important way that we can know God.