Existentialism is, essentially, a philosophical idea which rejects the authority in governments and religion (God), rather placing beliefs and ethics in the individual/themselves as existentialists. Put simply; to project the superiority of God or any deity whatsoever, and choosing to believe that they exist by themselves and make their own moral judgments irrespective of any outside spiritual governing influences. It could almost be considered as a fore-runner to humanism (leading to the question "What is the difference between humanism and existentialism?" see below); which is the more popular modern influence. Popularized by European writers post war in 1940s and 50s, existentialism emerged through literary works & basically opposed traditional philosophical thoughts existent at the time about human existence. The term "existentialist" is used very loosely; as most had disagreed in many points, and some rejected the validity of such a name.
On the existential view, to understand what a human being is it is not enough to know all the truths that natural science - including the science of psychology - could tell us.
The non-reductive dualist is no better off in this regard than is the physicalist. Nor will it suffice to adopt the point of view of practice and add categories drawn from moral theory: neither scientific nor moral inquiry can fully capture what it is that makes me myself, my "ownmost" self. Without denying the validity of scientific categories (governed by the norm of truth) or moral categories (governed by norms of the good and the right), "existentialism" may be defined as the philosophical theory which holds that a further set of categories, governed by the norm of authenticity, is necessary to grasp human existence. To approach existentialism in this categorical way may seem to conceal what is often taken to be its "heart" (Kaufmann 1968:12),