Søren Kierkegaard was a Danish philosopher and theologian, widely regarded as the father of existentialism. His work emphasized individual existence, personal choice, and the subjective experience of faith, which laid the groundwork for later existential thinkers. Kierkegaard's focus on the importance of personal responsibility and the angst of making choices reflects the core themes of existentialism, highlighting the struggle to find meaning in a seemingly indifferent universe.