Søren Aabye Kierkegaard was a Danish theologian, philosopher, poet, social critic, and religious author, widely considered to be the first existentialist philosopher. Born on 5 May 1813 in Copenhagen, Denmark, he passed away on 11 November 1855.

Kierkegaard wrote critical texts on organized religion, Christianity, morality, ethics, psychology, and the philosophy of religion with a fondness for metaphor, irony, and parables. He emphasized the importance of living as a "single individual" and prioritized concrete human reality over abstract thinking, highlighting personal choice and commitment.

His theological work focused on Christian ethics, the institutional Church, the differences between objective proofs of Christianity, the infinite qualitative distinction between man and God, and the individual's subjective relationship to Jesus Christ, which he believed came through faith. He was critical of Christianity as a state-controlled religion, particularly in the Church of Denmark. His psychological work explored the emotions and feelings individuals face when making life choices.

Unlike atheistic existentialism, Kierkegaard focused on Christian existentialism. His early work, often written under pseudonyms, presented distinctive viewpoints in complex dialogues. He left the task of discovering the meaning of his works to the reader, believing that "the task must be difficult, for only the difficult inspires the noble-hearted."

Kierkegaard's key ideas include subjective and objective truths, the knight of faith, recollection and repetition dichotomy, angst, the infinite qualitative distinction, faith as a passion, and the three stages on life's way. Though his work was initially limited to Scandinavia, it later gained substantial influence on philosophy, theology, and Western culture by the mid-20th century.