This is “Problem II” in Fear and Trembling, and it takes its inspiration from Kant’s notion that a divine command overriding the obligations of ethics is a logical impossibility: one would always be more justified in doubting the supposed divine command than in disregarding the ethical obligation. This is basically David Hume’s argument against miracles transposed into a moral, and almost existentialist key.
“There is an absolute duty to God, for in this relationship of duty the single individual relates himself as the single individual absolutely to the absolute…If this duty is absolute, the ethical is reduced to the relative” (p. 61).
“The tragic hero resigns himself in order to express the universal; the knight of faith resigns the universal in order to become the single individual” (p. 66).
On the man of faith: “Humanly speaking, he is mad and cannot make himself intelligible to anyone…If he is not viewed in this way, he is a hypocrite” (p. 67).