The main point of the existentialist argument is "existence over essence". Existentialist reject the abstract ways in which the rationalists look at humanity. They argue that by looking at Humanity from a rationalist perspective the existing realities are never understood. Existentialists also reject the idea of a universal principle that governs our behavior. Instead, they feel that what we do determines who we are, and not the other way around. This idea places an immense amount of responsibility on the individual. The individual is solely responsible for everything that happens in his or her life, including war and even to a certain extent their own birth. According to Satre this responsibility equates to and is integral to the idea of freedom.
Sartre's existentialism relates directly to the issue of ethics in that it holds the individual and only the individual responsible for his or her actions, be they ethical or unethical. For Satre, to claim that there are natural laws, or a universal principle, or an a priori set of guidelines that determine our actions is a delusional alleviation of our responsibilities. As far as Sartre is concerned, there is no such thing as a human nature, only a human condition.
Kierkegaard's philosophy underscores Sartre's idea that who we are is determined by the decision we make, not by some universal principle of human nature. He adds that not only do our actions determine character, but they also determine the quality of that character. It is important for the individual to make careful decisions so as to avoid regretting them at a later time. Kierkegaard draws the distinction between the ethical choice, and the aesthetic or unethical choice. He contends that the ethical choice is "absolute and strict, constituting the basis for finding oneself" (176); while he feels that the unethical or aesthetic choice is one made purely in the name of pleasure. The aesthetic choice is always one of shallow, instant gratification; while the ethical choice is for the better of the individual's inner personality. The ethical choice is made purely out virtue, not for our personal benefit, and not for the sake of conforming to certain rules.
Both Sartre and Kierkegaard hold the individual entirely responsible for his or her own actions. Not human nature, not social or natural order, not the influence of others are responsible for our actions. They agree that our actions determine our character, not that our character determines our actions. They are similar in that they reject looking at the essence of humanity in favor of looking at the existence of humanity.
One factor that naturally leads their ideas to clash is that one is an atheist and the other a theist. Kierkegaard argues for the existence of a God contending that such is an issue of faith, not of reason. Kierkegaard invokes the Old Testament story of Abraham to make his argument about the existence of God. Sartre later criticized this, asking how it was possible for Abraham to know that what he heard was the voice God and not the power of his own hallucination that spoke to him.