Kierkegaard apologizes that some readers may find his book "strange," since it appears to lack the seriousness one would presumably expect from a book on spiritual matters. However, it is in fact scholarly treatments that lack the appropriate manner. Whereas science and scholarship provide information about history and the world of objects and facts, "Christianity" is concerned with the spiritual well-being of individual human beings. Christian writing should therefore speak directly to the individual, even if this means adopting a less formal style.
Christian writing should adopt the manner of a "physician at a sickbed." "Despair" is the sickness in question, and the "cure" for despair is "to die unto the world"--that is, to adopt a spiritual outlook.
The irony and sarcasm of the Preface is typical of Kierkegaard's writing style. Rather than make straightforward arguments in support of his position, he often proceeds by ridiculing opposing views. His main target is science and historical scholarship. Scholarly and scientific writing pompously claims to offer unambiguous, "objective" facts. In Kierkegaard's view, it misses the point, since the issues of greatest concern to living human beings are not facts about the external world; they are spiritual matters that people must deal with privately.
The Preface sets up the major theme of the book, "despair." Despair is the "sickness unto death" referred to in the title. The point of The Sickness Unto Death is to demonstrate that "faith" is the way to overcome despair. Just what Kierkegaard means by despair--and by faith--will become more clear as the book advances.
Note: In some translations, the Preface refers to "upbuilding" as the purpose of Christian writing. While "upbuilding" offers a literal translation of the Danish word Kierkegaard uses, this word could also be translated as "edifying." Kierkegaard's point is that Christian writing should contribute to spiritual development.