Existentialism has been described as not so much a form of philosophy as a style of philosophising. It was extremely popular in the decades after World War II. The themes that were central to this kind of thinking fitted in with the mood of the times. Existentialism explored questions about human freedom, the absurdity of human life, alienation and the absence of a rational meaning in the universe. However, Existentialism was not one particular philosophical system that was shared by all those who are called existentialist. There are many varieties of existentialist philosophers, some are atheists, and others believers, both Catholic and Protestant. One of the most influential and best known was the French intellectual Jean-Paul Sartre.

Sartre was born in Paris on 21 June 1905. His father, a Catholic, was a naval officer who died when his son was still very young. His mother, a Protestant, was a cousin of the famous Albert Schweitzer, much of whose life was spent as a missionary doctor doing medical work in West Africa, in what is now Gabon. As a student, Sartre became interested in philosophy. In 1924 he met fellow student Simone de Beauvoir and they became lifelong lovers although they were never monogamous. Together they challenged the assumptions and expectations of their bourgeois upbringing. In the 1950s and 60s Sartre promoted Marxist ideas and tried to reconcile existentialism and Marxism. Later he condemned the Russian invasion of Hungary and Czechoslovakia. His ideas are expressed not only in his formal philosophical writings but also in essays, in his many plays, in the collection of short stories entitled The Wall, and in his novels Nausea and the trilogy collectively entitled The Roads to Freedom. In 1964 he was awarded the Nobel Prize for literature but refused to accept it. After 1973 he became almost totally blind and suffered a series of strokes. He died on 15 April 1980 and his funeral was attended by 50,000 people.

It has often been said that the existentialists are primarily concerned with the human person. This is certainly true of Sartre whose main aim was to understand human existence. For him the central problem always was to provide an existentialist account of what it is to be human. His early writings include studies such as The Imagination and Sketch for a Theory of the Emotions.

In Nausea he seeks to convey the meaning of the existence of things by analysing the nausea that overcomes its central figure one day while he is sitting in a park. He is impressed with the superfluity of all things, including even his own self. Everything shares in a kind of absurdity which characterises everything in the world. He is reduced to feeling sick and nauseous because there is a total absence of meaning to existence. Sartre is saying that if we reflect on the nature of the world around us the feelings we will experience are nausea, a sense of the absurd and anguish.

What characterises Sartre's philosophy above all is its outspoken atheism. In his lecture Existentialism is a Humanism he declares, "Existentialism is nothing else but an attempt to draw all the consequences from a consistent atheist position." His major philosophical work Being and Nothingness is considered to be the best and most sustained French exposition of an atheistic philosophy of existence.

One of the most important conclusions Sartre draws from his atheistic starting point is that since there is no God there is no universal moral law that human beings are obliged to observe and therefore no fixed set of values. In his lecture on humanism he quotes the famous saying of the Russian novelist Fyodor Dostoyevsky to the effect that if there is no God then everything is permitted and remarks that this is existentialism's starting point. If there is no moral law in accordance with which human beings ought to act then human liberty is unrestricted. Each individual person is the source of his or her own moral law. The individual alone is the source of the values according to which he or she decides to act. Without God the individual can only rely on itself. The human person is wholly free. This means that whatever he does is the result of his own free choice and what he becomes as a result of his choices depends entirely on himself.

Another aspect of his atheism is summed up in the phrase "existence precedes essence". As Sartre understands these terms they are intended to deny that things, particularly human beings, are created by God. Again, this means that individuals are absolutely free to create themselves. There are of course, major difficulties with this conception of an absolute human freedom. In his usage the term "freedom" is emptied of all meaning precisely because it does not apply only to the area of deliberation and free choice.

Human relationships, relationships of cooperation, of living together in a civilized manner, of friendship and especially of love, are a very significant aspect of human life. But given Sartre's presumption that God does not exist, it is not surprising that he has some strange views on the nature of relationships between people. Every relationship with another person is seen as a form of conflict. Conflict is the law that rules any contact between free individuals. The human person, in his view, must always remain an isolated and frustrated being. Hence a harmonious and enduring relationship based on love becomes only a futile ideal.

A positive result of considering an atheist philosophy such as that propounded by Sartre is that one has a good indication of the horrific nature of a world without God. A world without God would indeed be an absurd universe and a realm devoid of meaning. What Sartre has done is to show what the consequences would be if God did not exist. What he has not succeeded in doing is to show that philosophy must begin from the presumption that God does not exist.