What is the theme in Existentialism by ?

Atheism is a central theme within the philosophy of existentialism. Existentialism requires that one abandon any belief in God, because the concept of God contradicts the idea of personal responsibility that is at the center of the movement. Jean-Paul Sartre, the most prolific existentialist writer, was a fervent atheist, as were Simone de Beauvoir and Albert Camus. The characters in their novels can be seen as people coping with the loss of the concept of God by trying to determine the proper behavior in His absence.