Nothing in Kierkegaard's life (1813-1855) suggested he would enjoy posthumous fame. A peculiar man, often surly and unpleasant, possibly somewhat hunchbacked, Kierkegaard divided his time between wandering the streets of Copenhagen and writing his unusual philosophical books. He lived off a large inheritance from his father, he published his works at his own expense, and he wrote almost exclusively in Danish (his native language), attracting no readers of any significance outside his native land.
Though many of Kierkegaard's works were prepared as responses to popular ideas or influential writings in Denmark, few of his contemporaries appear to have taken his philosophy seriously. Towards the end of his life, a Copenhagen newspaper published a series of satirical cartoons about Kierkegaard that effectively reduced him to a local laughingstock.
Nevertheless, through a complex chain of events, Kierkegaard's works came to be read with interest by leading philosophers outside of Denmark early in the twentieth century. His fans came to include such notable figures as Wittgenstein, Heidegger, Kafka, Barth, Sartre, and Camus. These thinkers often interpreted Kierkegaard as a precedent for their own ideas--and therefore as an important step in the history of Western philosophy.
Within the context of the history of philosophy, Kierkegaard has generally been understood as a radical critic of Hegel. In brief, Hegel argued that we can obtain knowledge about religious and ethical truth through careful analysis of the historical process that created our ideas about religion and ethics. Kierkegaard argues instead that knowledge about the external world is always uncertain by comparison with our internal intuitions about God and morality. He urges us to pursue a private faith in God.
Ironically, while Kierkegaard's ideas have influenced twentieth-century theology, they have also been an important influence on thinkers who do not believe in God. For instance, "existentialist" philosophers like Sartre and Camus admire Kierkegaard's commitment to personal beliefs but reject his commitment to religion. (See the Overall Analysis and Themes for more on this and other interpretations of Kierkegaard.)
The Sickness Unto Death was published in 1849, just six years before Kierkegaard's death in 1855. It is one of Kierkegaard's last philosophical works, and it offers one of the clearest and most concise statements of his views on religious faith.