The Concept of Anxiety
All of Kierkegaard's books have either a preface, dedication, or prayer at the beginning. This book includes a lengthy introduction. "The Concept of Anxiety" was published on exactly the same date as "Prefaces", June 17, 1844. Both books deal with Hegel’s idea of mediation. Mediation is a common thread throughout Kierkegaard’s works. His work up to this point was to show that faith was being mediated by knowledge. Here he takes up the questions of sin and guilt.
Paul Tillich
Two of Tillich's works, "The Courage to Be" (1952) and "Dynamics of Faith" (1957), were read widely, including by people who would not normally read religious books. In "The Courage to Be", he lists three basic anxieties: anxiety about our biological finitude, i.e. that arising from the knowledge that we will eventually die; anxiety about our moral finitude, linked to guilt; and anxiety about our existential finitude, a sense of aimlessness in life. Tillich related these to three different historical eras: the early centuries of the Christian era; the Reformation; and the 20th century. Tillich's popular works have influenced psychology as well as theology, having had an influence on Rollo May, whose "The Courage to Create" was inspired by "The Courage to Be".