In Ethics , Bonhoeffer discusses the relation of the “ultimate” to the “penultimate,” God to the world, grace to nature. He admits that being man and being good are “penultimate in relation to the justification of the sinner by grace.” But this doesn’t mean that the penultimate conditions or determines the ultimate. On the contrary:
“it would be quite wrong, it would be robbing the ultimate, if we were to say, for example, that to be man is a precondition of justification by grace. On the contrary, it is only on the basis of the ultimate that we can know what it is to be man, so that manhood can be determined and established through justification.” True, “manhood precedes justification” and “from the standpoint of the ultimate it is necessary that it should precede it.” What validates the penultimate is not itself, however; what validate the penultimate is the ultimate.
In short: We don’t know man until we know the Son become man, the Son judges as man in the flesh, the Son raised to new life in the Spirit. We don’t know nature until we know grace because nature has not been defined until grace determines it.