Kierkegaard was a very religious man, but existentialism in the 20th century became more and more atheistic. Most of its main thinkers and writers were in mainland Europe. For example, Jean-Paul Sartre spent most of the Second World War in a German prison camp, reading the philosophy of Martin Heidegger. When he came out he gave a lecture called "Existentialism and Humanism". This early lecture may be easier to read than his later work.