Calvin takes as a basic axiom that the knowledge of ourselves and our knowledge of God is the most important aspect of knowing in general – of religious epistemology. They are tied together in an intimate way, Calvin says – from our knowledge of ourselves, we’ll come to a knowledge of God. However, no one can come to a true knowledge of themselves without a true knowledge of God. The two are tied together intimately.
Calvin treats knowledge of God first, and descends to the self later on – from the knowledge of ourselves, we can deduce only that we’re in a world of misery, and that we desire to rest in something greater. Our natural knowledge of God is suppressed or ignored, despite there being a kind of imprint of the divine on the heart and mind (this is an axiom for Reformed epistemology). The basic point, however, is that the knowledge of the self is tied together with knowledge of God – in Calvin’s case, our self-knowledge is knowledge of our need for God. For a brief exposition, see: http://thegospelcoalition.org/blogs/kevindeyoung/2010/01/15/a-calvin-clarification/
What happens, according to Bonhoeffer, is that when we begin to think and reflect upon ourselves, all our knowledge becomes self-knowledge, and takes part in the disunion of the knowledge of good and evil:
‘Man knows good and evil, but because he is not the origin, because he acquires this knowledge only at the price of estrangement from the origin, the good and evil he knows are not the good and evil of God but good and evil against God.’ (‘Ethics’, p. 23)
Knowledge of good and evil is the cause of all the disunion in man’s life – and the cause of death in man’s life. The problem isn’t that we know good and evil, and try to do the good, but fail, and see that we need God – the problem is that we know good and evil, apart from God, against God. In Bonhoeffer’s terminology, we become a god against God:
‘…man, knowing of good and evil, has finally torn himself loose from life, that is to say from the eternal life which proceeds from the choice of God…Man knows good and evil, against God, against his origin, godlessly and of his own choice, understanding himself according to his own contrary possibilites; and he is cut off from the unifying, reconciling life in God, and is delivered over to death. The secret which man has stolen from God is bringing about man’s downfall.’ (‘Ethics’, p. 23-24)
Bonhoeffer, contra Calvin, says that the knowledge of the self isn’t tied to knowledge of God – knowledge of the self is born out of disunion with God. This is an inversion of what’s a fairly key point in a lot of philosophy and theology – the knowledge of the self.