My take on Bonhoeffer's speculation in the context of SKs Knight of Faith is that although one lives in such a way as to seem divorced from the ultimate, in the sense that one does not focus on it, nevertheless, the ultimate will only be known in and through the act of living in the penultimate, a task the KoF addresses with joy and contentment.But we are still stuck with the concepts of ultimate and penultimate, and I want to see if we can unstick ourselves from those.
The sense of that boundary, of course, goes back at least to Augustine. He didn't originate it, but he gave it its strongest argument, basing it as he did on his perception of the flesh as the obstacle to the soul's contemplation of God (the "ultimate," for Bonhoeffer; the telos of the soul for Plato). This is, in turn, no accident, as Augustine is in some ways the most "Platonic" of the Church Fathers, and Plato all but argues for complete asceticism in the Phaedo (a point often lost in drawing lessons from this dialogue is that without the body and the world, the soul can never recover the knowledge it needs to escape the wheel of reincarnation and ascend to the realm of the Good.) Perhaps that position was forced on him by his need to refute Manichee. No matter, because the question is: is the sense of boundary really so critical to our sense of God? Because focus on boundaries is what leads to exclusion, and exclusion is not the experience of the Christian mystics (whereas Plato's hypothesis of the contemplation of the good is not the experience of anyone since, like death itself, that is a bourne from which no traveller returns.). The mystical experience is that focus on God eradicates boundaries, much as it did for Merton on that street corner. So, while taking up Plato's Hellenism within Christian theology, we have reason to lay it down again almost immediately.
The Knight of Faith does seem to live in the penultimate, in anticipation of the ultimate, but the problem is the Knight of Faith does something quite impossible for the rest of us: he relinquishes despair by grasping completely the thorn of despair. He yields completely the notion of grasping the ultimate, and thereby, paradoxically, grasps it. Except he doesn't, of course. That's the purpose of de Silentio's meditation of the akedeh of Abraham. The individual grasps eternity; but eternity is the one thing we cannot grasp at all.
Abraham sacrifices Isaac fully, says de Silentio. He lets go of Isaac before the knife comes down. If not for the angel stopping his hand, Isaac's life would be taken from him. Isaac is bound following the ritual for binding a sacrificial animal, the akedeh. Isaac's life is already offered to God before Abraham can raise the knife. All that is left is to complete the deed, and only in that is he stopped. God, as was said to Jeremiah, tests the heart to find out what is in it. In Abraham's heart, Isaac is gone. How does he ever get his son back?
That's the question of the teleological suspension of the ethical, the context in which the entire discussion of Fear and Trembling takes place. And that very issue, the issue of the effect of the ultimate (in Tillich's sense of ultimate concern, or source of being) being known in human existence, is the mysterium tremendum that shakes an individual to her very core, and which never stops shaking it. Do we ever, in other words, reach a stage of harmony with such a condition? Or are we left in the state of fear, and trembling?
The philosophical character, de Silentio, is pursuing that question from the outside, looking in. He describes the ideal person who would represent the telos he is seeking. But that person is not a mystic, not a contemplative, not apparently more religious than anyone else, at all. That person is not even Abraham, because even Abraham was uprooted from his world by the call of God, and flung headlong into the unknown; flung into the unknown, in fact, over and over again. Even the connection between Abraham and the Knight of Faith is an ironic one. It is philosophy undermining reality, trying to reduce it to a system that can be contained and comprehended within a narrow sphere. One can never read Kierkegaard without being aware of his antipathy for the systematizing of Hegel, and his awareness of the corrosive irony of Socrates (an acid which ultimately destroys all and leaves not even itself behind).
This doesn't quite get us to left rev.'s point (you will have noticed), that we only know the ultimate through living in the penultimate. But we're at least slouching toward that goal. In the meantime, we have another question from the left rev. to answer: "If we had no eschatology, would we have need of a soteriology?" So, unless we are distracted again, that's where we will go next.