Written by Danish philosopher Soren Kierkegaard and published under a pseudonym in 1849, The Sickness Unto Death is a radical exploration of Christianity, specifically Christian existentialism. Kierkegaard is widely considered to be the founder of existentialism, examining the concepts of despair and individualism in great detail in his works.
The Sickness Unto Death first and foremost is about despair. Kierkegaard regards despair as one and the same as the Christian concept of sin – the original human sin. Throughout the book, Kierkegaard classifies the various forms and despair and its sinfulness. He states that despair is one placing limitations on oneself, and to exist without despair is to free one's mind and be aware of the universe's infinite power.
The Sickness Unto Death is a prominent work of Christian existentialism, often cited as one of the first works to have truly explored its concepts. It remains an important and influential piece of literature, used in many philosophy classes to discuss religious existentialism.