Soren Kierkegaard was a 19th-century Danish philosopher, theologian, and author widely considered to be the first existentialist thinker. He focused on the importance of individual existence, subjective truth, and personal faith, challenging the established religious and philosophical systems of his time. Kierkegaard's work explored themes such as anxiety, despair, and the nature of authentic Christian faith, emphasizing the importance of personal choice and commitment in one's relationship with God.