From the perspective of an unbeliever, Fear and Trembling explores the paradox of faith, the nature of Christianity, and the complexity of human emotion. Kierkegaard examines the biblical story of Abraham, who was instructed to sacrifice his son Isaac, and forces us to consider Abraham's state of mind. What drove Abraham, and what made him carry out such an absurd and extreme request from God? Kierkegaard argues that Abraham's agreement to sacrifice Isaac, and his suspension of reason, elevated him to the highest level of faith. He explores more comprehensible alternatives, but in each one Abraham fails the test of faith, thus showing that true faith cannot be explained, understood, or made rational.

His thesis is a compelling counterpoint to Hegel, who maintained that reason was the highest form of thought, and it proved a significant source of inspiration to later existentialist philosophers such as Camus and Sartre.