Many 20th-century philosophers, both theistic and atheistic, and theologians drew concepts from Kierkegaard, including the notions of angst, despair, and the importance of the individual. His fame as a philosopher grew tremendously in the 1930s, in large part because the ascendant existentialist movement pointed to him as a precursor, although later writers celebrated him as a highly significant and influential thinker in his own right. Since Kierkegaard was raised as a Lutheran, he was commemorated as a teacher in the Calendar of Saints of the Lutheran Church on 11 November and in the Calendar of Saints of the Episcopal Church with a feast day on 8 September.
Philosophers and theologians influenced by Kierkegaard are numerous and include major twentieth century theologians and philosophers. Paul Feyerabend's epistemological anarchism in the philosophy of science was inspired by Kierkegaard's idea of subjectivity as truth. Ludwig Wittgenstein was immensely influenced and humbled by Kierkegaard, claiming that "Kierkegaard is far too deep for me, anyhow. He bewilders me without working the good effects which he would in deeper souls". Karl Popper referred to Kierkegaard as "the great reformer of Christian ethics, who exposed the official Christian morality of his day as anti-Christian and anti-humanitarian hypocrisy". Hilary Putnam admires Kierkegaard, "for his insistence on the priority of the question, 'How should I live?'". By the early 1930s, Jacques Ellul's three primary sources of inspiration were Karl Marx, Søren Kierkegaard, and Karl Barth. According to Ellul, Marx and Kierkegaard were his two greatest influences, and the only two authors of which he read all of their work.
Kierkegaard has also had a considerable influence on 20th-century literature. Figures deeply influenced by his work include W. H. Auden, Jorge Luis Borges, Don DeLillo, Hermann Hesse, Franz Kafka, David Lodge, Flannery O'Connor, Walker Percy, Rainer Maria Rilke, J.D. Salinger and John Updike. What George Henry Price wrote in his 1963 book The Narrow Pass regarding the "who" and the "what" of Kierkegaard still seems to hold true today: "Kierkegaard was the sanest man of his generation....Kierkegaard was a schizophrenic....Kierkegaard was the greatest Dane....the difficult Dane....the gloomy Dane...Kierkegaard was the greatest Christian of the century....Kierkegaard's aim was the destruction of the historic Christian faith....He did not attack philosophy as such....He negated reason....He was a voluntarist....Kierkegaard was the Knight of Faith....Kierkegaard never found faith....Kierkegaard possessed the truth....Kierkegaard was one of the damned."
Kierkegaard had a profound influence on psychology. He is widely regarded as the founder of Christian psychology and of existential psychology and therapy. Existentialist (often called "humanistic") psychologists and therapists include Ludwig Binswanger, Viktor Frankl, Erich Fromm, Carl Rogers, and Rollo May. May based his The Meaning of Anxiety on Kierkegaard's The Concept of Anxiety. Kierkegaard's sociological work Two Ages: The Age of Revolution and the Present Age critiques modernity. Ernest Becker based his 1974 Pulitzer Prize book, The Denial of Death, on the writings of Kierkegaard, Freud and Otto Rank. Kierkegaard is also seen as an important precursor of postmodernism. Danish priest Johannes Møllehave has lectured about Kierkegaard. In popular culture, he was the subject of serious television and radio programmes; in 1984, a six-part documentary Sea of Faith: Television series presented by Don Cupitt featured an episode on Kierkegaard, while on Maundy Thursday in 2008, Kierkegaard was the subject of discussion of the BBC Radio 4 programme presented by Melvyn Bragg, In Our Time. Google honoured him with a Google Doodle on his 200th anniversary.
Kierkegaard is considered by some modern theologians to be the "Father of Existentialism." Because of his influence and in spite of it, others only consider either Martin Heidegger or Jean-Paul Sartre to be the actual "Father of Existentialism." Kierkegaard predicted his posthumous fame, and foresaw that his work would become the subject of intense study and research. In 1784 Immanuel Kant, many years before Kierkegaard, challenged the thinkers of Europe to think for themselves in a manner suggestive of Kierkegaard's philosophy in the nineteenth century. In 1851 Arthur Schopenhauer said the same as Kierkegaard had said about the lack of realism in the reading public in Either/Or Part I and Prefaces. In 1854 Søren Kierkegaard wrote a note to "My Reader" of a similar nature.