Despair
Despair is the key theme of The Sickness Unto Death; Kierkegaard's main purpose in the book is to inform readers of the natures of their despair, which he attributes to ignorance of reality and the world's infiniteness. Throughout the book, he equates despair with sin, and discusses how to free the mind of despair and therefore sin.
Sin
In The Sickness Unto Death, Kierkegaard equates sin unequivocally with despair, for despair is a form of ignorance and limitation. Kierkegaard stays true to the idea of Christian sin by stating despair is the original sin, but also re-invents it by stating that despair is a basic human trait, though one that must be eradicated to unbind the mind.
Individualism
Individualism is an extremely important theme throughout Kierkegaard's works. Specifically in The Sickness Unto Death, Kierkegaard discusses how despair limits self-expression and self-awareness, which in turn limits individualism. He describes individualism as ultimate form of freedom, to have thought independent of others.
