Sartre was not the first to elaborate a system of existentialism. The question of existence has preoccupied humankind since its beginnings, and as early as the Bible, free will has been a major concern. One reads in Ecclesiasticus (15:14) (in the Apocrypha) that "God made man free in the beginning, and then left him free to make his own decisions." This notion of freedom is echoed down through the ages, from varying points of view. Rousseau's Social Contract (1762) revolves around his version of the above quotation ("Man is born free but is everywhere in chains"), and so do the works of the writers known to us as existentialists. The Greeks and Romans also had their own ideas concerning freedom, and it is still a burning issue in the writings of the twentieth century.
There are two broad categories of existentialists: Christian and atheistic. Among the Christians one finds Soren Kierkegaard, Karl Jaspers, and Gabriel Marcel; the most celebrated of the atheists, prior to Sartre, was Martin Heidegger.
Kierkegaard, a Danish philosopher of the nineteenth century, is generally regarded as one of the founders of modern existentialism. As a Christian, he claimed in his major work, Either/Or, that one could choose either a life with Christ or a life without Christ. He felt there were no logical reasons for believing in Christ and that a leap of faith — not rational proof of God's existence — was what constituted belief. He believed that, regardless of one's religious choices, humanity would be tormented by conscious or unconscious despair (see Sickness unto Death, 1849), and when one dwelled on this despair, it would become "dread" (see The Concept of Dread, 1844). This idea is significant as a precursor to the Sartrean notion of "nausea."
Kierkegaard's beliefs brought him into direct conflict with the ideas of Hegel, the German philosopher who claimed to have rationalized Christianity. Hegel subscribed to Kant's assertion that Jesus had originally taught a rational morality and offered a religion which was suitable to the reason of all people. Hegel argued that the history of humanity was the intelligible reflection of God's will, but Kierkegaard attacked this thinking on the basis that one would have to be God to know what God's will was (see Concluding Unscientific Postscript, 1846). Kierkegaard was not interested in an objective, scientific proof of God; he knew that life, regardless of religion, was colored with despair, and that a detached "objectivity" was unable to change this. He proposed that the only way truth could be found was through one's intuitions and feelings, through the leap of faith ("truth is subjectivity"). This form of existentialism, anchored in a belief in God, was not one with which Sartre could agree.
Closer to Sartre's thinking was that of Heidegger, the German philosopher whose works had a profound influence on the budding French writer. Heidegger's atheistic Sein und Zeit addressed the basic metaphysical question of "Why is there something rather than nothing?" This will figure later in the Sartrean position on existence preceding essence, especially given Heidegger's conclusion that life is woven with care and dread, and that all projects come to nothingness at death. It is worth noting that Sartre's major philosophical treatise, Being and Nothingness, bears the very clear mark of this mode of reasoning. Sartre took from Heidegger the basic notion that life was absurd, that the universe was irrational and meaningless, and that the ultimate absurdity was death. From these ideas, he moved into his own system of thinking and evolved what is now known as Sartrean existentialism.