Intellectual setting
Kierkegaard wrote in Golden Age Copenhagen, reacting against the dominance of Hegelian philosophy in Danish intellectual life (represented above all by the theologian Hans Lassen Martensen) and against what he saw as the spiritual complacency of the established Danish Lutheran church under Bishop J. P. Mynster. His broken engagement to Regine Olsen, his public feud with the satirical weekly "Corsaren," and his final "Attack upon Christendom" pamphlets in the year of his death were all woven into his authorship.
Key ideas
The stages (spheres) of existence
Kierkegaard distinguished the aesthetic, ethical, and religious as qualitatively distinct ways of relating to oneself and to existence, arguing that movement from one sphere to another requires a decisive, non-rational "leap" rather than gradual rational or dialectical progression.
Subjective truth and the leap of faith
Against Hegelian claims to objective, systematic knowledge, Kierkegaard held that the truths that matter most - especially religious ones - cannot be grasped through detached reasoning but only through passionate, subjective appropriation, most starkly dramatized by Abraham's paradoxical faith in "Fear and Trembling."
Legacy
Widely regarded as the father of existentialism, Kierkegaard decisively shaped Heidegger, Jaspers, Sartre, and Camus, as well as twentieth-century Protestant theology through Karl Barth and Rudolf Bultmann and later philosophy of religion. His experiments with pseudonymous, indirect communication and his critique of philosophical system-building remain central reference points in existentialist and continental philosophy.