Many people would describe Soren Kierkegaard as our first existentialist philosopher. He wrote about the topics of philosophy, Christianity, and psychology. In 1844 he published The Concept of Anxiety, where he used the pseudonym of Vigilius Haufniensis to explain the roots of anxiety.
In the book, a man stands at the edge of a cliff and sees all of life’s possibilities. As long as he stays there, he experiences the dizziness of freedom where everything, even the most frightening of things, is possible. Having the freedom to do anything he wants, he also experiences tremendous fear. Anxiety lies in the possibility of complete freedom.
Kierkegaard also describes man’s first experience with anxiety when Adam was confronted with the choice of eating from the tree of knowledge. Adam didn’t know good from evil. All he knew was that God had told him not to eat from the tree. By doing so, he had left Adam free to choose, which was the beginning of man’s anxiety.
According to Kierkegaard, the speculative philosophy of his time was useless. No one could determine the truth about our existence or God’s being. Man was therefore free to pick his worldview, something that led to great anxiety.