Some people, Kierkegaard tells us in the Preface, might expect books on religious matters to be serious and scholarly. Religious books should instead strive to engage the reader on a personal level. Religious writing, the Introduction explains, should adopt the manner of a physician at a sickbed. It should help people cure themselves of the "sickness unto death"--the fear that our lives will amount to a spiritual void rather than the eternal life that Christ promised.
Part I explains that "despair" is the "sickness unto death." Human beings are a "synthesis" of spiritual and physical elements, and despair is a misrelation between these elements. The solution to despair is a condition in which the individual has established a relationship with the "power that established it" (in other words, with God). People may appear to despair over facts of the world, but despair is in fact always an internal problem for which the individual is personally responsible.
Despair is universal. People may be in despair and not know it. People may despair through excessive imagination or through excessive concern about their material circumstances, through a sense of vast possibilities or a sense of lack of options. There is a hierarchy of forms of despair running from a weak desire not to be what one is to a "defiant" desire to be entirely self-sufficient.
Part II explains that, in Christian terms, despair is sin. Christ has revealed to us that faith is the solution to despair. Once we have received this revelation, it is a sin to neglect it and choose to remain in despair. Just as there is a hierarchy of forms of despair, so is there a hierarchy of forms of sin, ranging from indifference to a defiant refusal to accept religious truth. Sin may be intensified in complex psychological forms, such as despairing over sin (focusing obsessively on one's sinfulness), despairing over the forgiveness of sins (feeling that such forgiveness is not possible for one's sins), or, worst of all, despairing over Christ's teachings (dismissing Christianity as untruth).