Abraham's Biblical allegory
The Bible is an important part of this book of religious philosophy. The title of the book comes from Philippians 2 where Paul tells his reader to "Work out your faith in fear and trembling." Then, the book directly addresses the story of Abraham, the father of the Abrahamic religions and the "Father of Faith," as he is called in Hebrews 11. Kierkegaard calls him that too, writing about the story that set him apart from the rest of us, a strange tale where God told him to murder his own son as a blood sacrifice.
The acceptance of fate
Kiekegaard, writing under the pseudonym, "Johannes de Silentio," is concerned with the issue of human fate. He elaborates alternative responses that Abraham might have had, constructing a motif that might be called, "Aversion to one's fate," or "Refusal of the call." The question of fate is ultimately a response to the terror of existence, which he treats in "The Eulogy of Abraham." Fate and death are the ultimate philosophical considerations for a living human, because as Kierkegaard observes, death sets an ultimate intensity on the question of life, and fate is the idea that there is a specific goal to one's life.
Telos
The philosopher invokes a word from ancient Greek philosophy. A "telos" is the end to which something else is the means. In other words, when Jesus says, "It is finished," (in Greek the word is the verb form of the word "telos"), that isn't the chronological end of humanity, but it might be the ultimate "end" of humanity in a philosophical sense. That is the consideration that Kierkegaard elaborates, noticing that Abraham's teleological "end" was to fully commit to his fate, so that he becomes eligible of angelic salvation.
The allusion to Kant
Kierkegaard walks away from the Bible for a moment to have a dialogue with some of Kant's writings, and he talks about Kant directly, by name. He discusses the way Kant treats human existence in books like Critique of Pure Reason, in which book Kant famously observed that to automatically assume that human reason is objectively correct and perfect in perception is simply to beg the question. Kierkegaard's allusion allows him to piggy-back on this idea to state that in addition to human limitations, humans are also imbedded in a moral framework through time that has fateful implications.
Silence and "Silentio"
The penname "Johannes de Silentio" means "John of silence," or "Silent John." Then, in the final section of the book, the author raises a question about Abraham's silence. He says that he would not have been able to complete his mission if he had explained his intentions to his son, so the passion that it took him to do his fate made his perfectly self-contained, solipsistic, and silent. He decided a long time before the action that he was going to complete the calling. Silence is a symbol for absolute certainty, something that Johannes de Silentio likens to the Passion of the Christ. He calls silence a kind of passion, from Latin, "passus," meaning suffering. To suffer the loneliness of silence is an indication of one's conviction.
