Soren Kierkegaard was a Danish philosopher, theologian, and writer in the 19th century. He is known for his contributions to existentialism, exploring themes such as individuality, freedom, and choice. His works often delve into the complexities of faith, ethics, and the human condition.

Soren Kierkegaard.

Existentialism existed before the term was coined. The first thinkers who expressed significant existentialist ideas were Soren Kierkegaard and then Frederick Nietzsche. It gained more shape and definition in the works of Martin Heidegger, and came to more full expression with Jean-Paul Sartre and Albert Camus.

Short answer: Kierkegaard.

Soren Kierkegaard