One of the things I have been studying lately are the various prominent theologians of the 20th century and their thought. The last century saw a strong reaction against the liberalism of the 1800's (which sought to discard or reinterpret the old doctrines of orthodoxy where they seemed problematic to the modern mind), mainly (or at least initially) in the form of the so-called neo-orthodoxy or dialectical theology of Karl Barth and Rudolf Bultmann. These two men were at odds with each other in some significant ways - so much so that Barth accused Bultmann of reverting to the old liberalism they were attempting to demolish and replace - but have been included together under the common umbrella term of "dialectical theology" by historical theologians because of a stress from both men, contra liberalism, of the differences between God and humanity. While much more could be said about both men (which I may do later on), it is on this last point in regards to Bultmann's theology on the nature and knowledge of God that I would like to offer some comments.
Bultmann (1884-1976) was not a systematic theologian, but during his life as a career scholar he enjoyed a high standing among academia as a major New Testament scholar. After an initial foray into the dialectical theology of the 1920s, he became deeply influenced by existentialism (particularly that of Martin Heidegger) and allied this somewhat paradoxically with his deep interest in historical explanations and biblical studies to produce a brand-new restatement of the message of the New Testament - a sort of Christian existentialism. He is best known for his program of demythologizing, the task of restating Christianity in language that made sense to people with a modern, "scientific" worldview by stripping it of the outdated, "mythological" worldview in which it had first appeared. In other words, there was a core of theological truth contained in Scripture, but you had to get rid of the mythology (such as supernatural occurrences, expectations, and the worldview that made these possible) to find it and understand it as a modern. (Though motivated differently, this view in many ways put him in the same bed as the old liberals.)
This idea rested on what Bultmann saw as the most important message to be extracted from the New Testament: God. A great God far above and vastly different from the material world and the humans who inhabit it - so far beyond us, in fact, that there is no way we can know Him, or even speak sensibly of Him at all. God is not an object out in the universe at which we can point, an existing thing; He is the underpinning reality of every object and the whole universe, the basis of existence itself. This position was not new; theologically, it has roots in the Cappadocian Fathers of the East, Pseudo-Dionysius propounded it in the 6th century, and Thomas Aquinas produced probably the most highly developed and sophisticated version of this in church history. His view, from whence Bultmann takes his cue, is basically that God transcends everything, even our ways of thinking, to the point that no statement we make about Him can do Him justice - and because of this we can't really say anything about Him at all, at least not in the same way we could say something about a ball, or a chair, or my cousin Rob.
Bultmann recognized, as did Aquinas, that this view of God is problematic when one is attempting to do theology - that is, speaking of God. Aquinas found his way out by using analogy - in other words, when we say God is a certain thing (e.g., good, powerful, wise, etc.) we are just using analogical language, the best that we have, attempting, as accurately as possible, to describe the indescribable. Bultmann, however, took a different route:
If "speaking of God" is understood as "speaking about God," the such speaking has no meaning whatever, for its subject, God, is lost in the very moment it takes place. Whenever the idea, God, comes to mind, it connotes that God is the Almighty; in other words, God is the reality determining all else...Every "speaking about" presupposes a standpoint external to that which is being talked about. But there cannot be any standpoint which is external to God. Therefore it is not legitimate to speak about God in general statements, in universal truths which are valid without references to the concrete, existential position of the speaker.
Historian Jonathan Hill sums it up this way:
In other words, instead of trying to make objective statements about God, we should speak about our own subjective experience. Because our existence is dependent on God, understanding ourselves will allow us to understand him.
I certainly don't agree with Barth on everything, but I stand behind him in refutation of that point. We do not understand God by understanding ourselves. God has revealed himself to us through the person of Jesus and propositionally through His written Word. The Christianity of the apostles and martyrs is built not off of introspection, but upon revelation.
But even if Hill's isn't an accurate summation of Bultmann's point, a problem still remains. Do you see it? It is so fundamental that it shapes the rest of his errant theology. Every worldview begins with an ultimate reality, and out of that the rest flows. Every religion, every theology begins with a view of God, upon which the rest finds its logical base. It's sort of like using a level when beginning to build a house - mess it up there, and the whole thing will be off.
Let me break Bultmann's argument here down into a syllogism to aid in getting at the problem:
1) God is the reality determining all else.
2) Every "speaking about" presupposes a standpoint external to that which is being talked about.
3) But there cannot be any standpoint which is external to God.
4) Therefore it is not legitimate to speak about God in general statements, in universal truths which are valid without references to the concrete, existential position of the speaker.
The flaw in his reasoning can be seen in premise 2). It is patently and demonstrably false, and without this crucial premise, the thing won't fly. To show it is false, ask yourself these two questions: can you speak about yourself? If so, how could you be external to yourself?
The truth is, I can speak about myself. I can know objective things about myself, yet I am never once operating from a standpoint external to myself. Without this faulty premise, the conclusion is unwarranted. So for at least this consideration, the argument should be rejected. Moreover, one may be able to take issue with premise 3) by making the distinction between how one could be internal to God in one way (ground of existence) and external to him in another (personhood).
I call this (long) post "Bultmann's Fundamental Mistake" for this reason: his mistake here effectively drew the outline of the rest of his theology and seems to me to be the seed from whence his other errors sprung. This can serve as a healthy reminder to us to examine our fundamental worldview commitments - without an accurate level, the whole house becomes crooked and, in fact, dangerous.
 This conclusion, that a Wholly Other God is the important message conveyed by Scripture behind the mythology, really is untenable and rather absurd, given his own view. If this is the core of the "mythological" worldview, how is it that one can be left with this utterly and foundationally mythological element (given his definition of such things) after one has stripped away the myth? (In other words, it makes no sense to say that latex is the most important element of the wall next to me if I say that I need to remove the latex paint to get to it.) And how can something fundamentally inconsistent with a "scientific" worldview be stated sensibly in that worldview at all? This is a dead giveaway that his whole system is flawed.
 What Does It Mean to Speak of God? I
 The History of Christian Thought, pp. 280.