Each of the three problemata takes an ethical question that is supposed to be answered definitively by the Hegelian system and then shows how the story of Abraham contradicts the ethical question as answered. The preliminary expectoration is meant to set up the contrast between the ethical and the religious that will be exploited in the problemata that follow. Kierkegaard also has Johannes use the "preliminary expectoration" to introduce important terms, such as "anxiety," "absurd," "double movement," and "paradox." These terms are all treated in the glossary, but we might do well to touch upon them here as well.
"Anxiety" is a translation of the Danish angest, often rendered as "dread." Roughly speaking, it is a species of fear that has no specific object, but is caused rather by the dizzying awareness of one's freedom to define oneself by one's choices. Abraham's choices and deeds define him as a knight of faith, but set him directly in opposition to the dictates of the ethical life. The responsibility for these choices is entirely Abraham's, and in making them, he defines himself as a knight of faith. He feels anxiety as he faces the choice of whether to exemplify the religious or the ethical way of life.
In saying that Abraham has faith "by virtue of the absurd," Johannes says that Abraham's faith transcends all human and intelligible possibility. There is absolutely no intelligible way that Abraham will get Isaac back, and yet he maintains his faith. Faith goes beyond all reason and everything that can be said or understood. In that sense, faith is absurd: it cannot be explained and it cannot be justified.
Because both the religious and the aesthetic place their emphases on the single individual as opposed to the ethical's emphasis on the universal, there is a worry that the aesthetic may be confused for the religious. The aesthetic is easily attained, whereas the religious requires the "double movement" of faith. In order to exemplify the ethical, one must make the movement of infinite resignation, surrendering one's individual desires to the greater good of the universal. To have faith, one must go further and make another movement, a leap of faith into the absurd, where the individual again becomes important in a direct relationship with God. The religious is distinguished from the aesthetic because only the religious demands this double movement, whereby one renounces everything and then regains everything by virtue of the absurd.
A great deal of this commentary may seem very confusing, but to an extent this cannot be helped. The problem is that Fear and Trembling deals with faith, and asserts that faith is beyond reason, beyond understanding, beyond words. Thus, any attempt to understand it or to explain it is bound to fail. Herein lies a part of the paradox alluded to throughout the work. A paradox is a kind of contradiction, where one derives from certain premises the falsity of those premises. Fear and Trembling takes the Hegelian expression of the ethical as universal and Hegel's praising of Abraham as the father of faith as its premises and shows how the story of Abraham, if accepted as laudable, contradicts the ethics expressed in the Hegelian system. Abraham is a paradox because he cannot be made sense of, because he contradicts ethical principles. On an ethical level, Abraham is a murderer.
What does it mean to say that Abraham is a murderer? If one imagines watching Abraham, all the facts suggest that he is: he silently packs his bags, takes his son to a mountain, and draws a knife to kill him. The only justification for this behavior is that it was God's will, but God's will was expressed in a private covenant between God and Abraham. No one else knows what God said to Abraham, nor can Abraham explain it. Thus, no third party that tries to understand Abraham's behavior according to any kind of moral code will be able to justify what Abraham does.