Søren Kierkegaard was a Danish philosopher. He wrote about religion, or theology. Kierkegaard talked about anguish in several of his books: "Fear and Trembling" (1843), "Either/Or" (1843), "The Sickness Unto Death" (1849), and "Two Discourses of God and Man" (1938). In the book "Eighteen Upbuilding Discourses" (1843), Kierkegaard talked about Paul the Apostle. Paul felt anguish because he persecuted Christians.