In Fear and Trembling Kierkegaard introduces the "Knight of Faith" and contrasts him with the "knight of infinite resignation". The latter gives up everything in return for the infinite, that which he may receive after this life, and continuously dwells with the pain of his loss. The former, however, not only relinquishes everything, but also trusts that he will receive it all back, his trust based on the "strength of the absurd".
For Kierkegaard, infinite resignation is easy, but faith is founded in the belief in the absurd. The absurd is that which is contradictory to reason itself. For Abraham, this faith in the absurd manifests itself in Abraham's belief that he would kill his only son but he would nevertheless receive him again in his lifetime. Silentio's opinion is that what separates Abraham from being a killer is his faith. (In the end of the Genesis 22 story, an angel stops Abraham at the last moment. A ram appears which Abraham takes as a sign from God, and he sacrifices the ram instead of Isaac.)
An important theme is the conflict between theology and philosophy. According to Kierkegaard, mid-19th-century secular philosophers laughed at faith and saw no mystery in the story of Abraham while professing to find Hegel's philosophy exceedingly difficult. Kierkegaard, however, thought that understanding Hegel was possible (if difficult), but trying to comprehend why Abraham was willing to sacrifice his son caused him to be "virtually annihilated".
The book was referenced on LA X, an episode of Lost.