Existentialism is a misunderstood philosophy. It tends to be the “philosophy” of teenage nihilists, rebellious individualists, and other alienated persons who take a liking to the theme of alienation in existentialist thought. However, existentialism is not a nihilistic philosophy; on the contrary it attempted to confront the crisis of nihilism. Moreover, despite the “individualist” element to it, existentialist individualism (in its Sartrean or Camusian form) is a tragic individualism that does not celebrate the primacy of the individual but laments it. But such individualism often misses the point—Heideggerian authenticity is not a celebration of atomistic individualism either; anyone who has read Being and Time would know Heidegger’s ontic insight of Being-in-the-World means Being-With, a being-in-relationship to world and others.
The Christian roots of existentialism are also well-attested to in scholarship. This has nothing to do with Kierkegaard but everything to do with the Christian anthropological concepts of “The Fall of Man,” “Original Sin,” and Augustinianism. In his short but concise summation of existentialism, William Barrett outlined how existentialism had Christian roots going back to Tertullian and St. Augustine. The reader of Christian theological anthropology will immediately recognize many of the same themes in existentialist thought though in a now “secularized” form: The crisis of love and wanting to love and be loved; alienation from self and from the world the self exists in; trying to find happiness but ending up engaging in domineering behaviorism; seeking rationality because man is a “rational” animal.
The difference between existentialism—at least in its atheistic form—with Christianity is that it embodies Augustinian tragedy without hope of salvation. To some this is the “grown up” version; that Christianity necessarily leads to tragic existentialism because there is no God to reconcile these dilemmas for us. This was, at least, Heidegger’s understanding of the present moment existentialism sat in. As such, existentialism superficially seems to be “friendly to paganism” because of the pagan fatalism of the heroic hero who struggles against the odds of the world because that is all he is capable of doing.
Therefore, the impetus of existentialism is the recognition that life is defined by some sort of worrying characteristic or characteristics. There is a certain “original guilt” or “anxiety” that looms over our lives which we are, to varying degrees, aware of. Humans have anxiety about themselves, their lives, the world they live in. They seek to be themselves but fail to be themselves. They seek meaning in a meaningless world.
Those who get their knowledge of existentialism from Wikipedia receive a deeply misleading and, at times, wrong, understanding of existentialism. It is true that the “authentic self” is a concept sought after in existentialism, with differing understandings of what that means pending the individual existential philosopher: From the rootedness of Heidegger to the pious struggler of Camus to the being-for-itself of Sartre. Moreover, the Augustinian roots of existentialism are best seen in existentialist anthropology emphasizing the primacy of will (voluntarism) and being responsible for the actions they engage in (“sin has consequences” becomes “freedom has consequences”).
As Camus argued, the only question of philosophy is the question of life. Life is the only question philosophy concerns itself with because life is contingent to being; without being there is no life. Without life there is no existence. Without existence there is nothing to ponder since subjectivity is contingent upon existence. Thus, the question of life is the question of being—the same question that Heidegger attempted to answer in Being and Time.
Camus posited the view that we live in a cold, irrational, unordered, unloving, and meaningless world. This facticity was at odds with man’s subjective desires. His desire for goodness, love, rationality, and meaning. The two come into conflict. In essence, it is the rehashed subject-object dialectic of earlier Christian and Romantic philosophy.
Part of the Christian doctrine of the Fall is man’s alienation from the world (represented by his expulsion from Eden), man’s alienation from himself (in rejecting his rationality and favoring his pure desire), man’s alienation from others (represented by Adam’s ruptured relationship with Eve when he blames God for having created Eve who made him eat from the tree), and man’s alienation from God (the source of Truth and happiness). As such, man is alienated in the world. He seeks the good and true but is unable to live by the good and true. He seeks relationships with others but is alienated from them, often leading to the lust for domination (libido dominandi). He seeks happiness but only makes himself miserable. He does not use his rational soul to conform to the standards of nature but rationalizes his actions to make his “sin” seem acceptable. Man, in essence, does not want responsibility for what he is.
Existentialism plays off of these themes. It accepts, at face value, the alienation of man. It equally asserts that man is at odds with nature (e.g. he seeks meaning and order in an unmeaningful and unorderly world). Man does not seek responsibility for his life because such freedom is burdensome. As such, man is perpetually alienated.
Existentialism, in its many forms, saw this dilemma playing itself out in three ways. The first two were bad. The last “good.” Of the first two roads this alienation leads man down to was either suicide (the coward’s way out) or “bad faith” (refusal to accept reality; the ignorant man’s way out). Nietzsche used the term “Last Man” for the equivalent of the man in bad faith. Hegel called such a person “the victim of history.” The alternative to suicide or bad faith was “authenticity.”
But what exactly was authenticity? Again, authenticity varies from philosopher to philosopher. What one can say about authenticity is that it lies with coming to terms with the fundamental nature of the world, embracing it for what is, and living by the standard of nature (whoever construed). Only in this way could alienation be confronted, and from this confrontation authentic meaning found.
For Nietzsche this meant the embrace of the eternal struggle for life itself in its quasi-Darwinian battle for evolutionary progression. The Cosmos, for Nietzsche, was a giant battlefield where the struggle for life played itself out. For Camus it was the embrace of the very nature of the absurd; piously accepting the absurd reality of the world and not allowing it to crush you into defeatism. For Sartre it was perpetually choosing to live for yourself rather than for others, since living a life for others cannot reconcile our alienation from others.
Existentialism was the culmination of the anti-nihilist tradition of continental and romantic thought. Existentialism is not, per se, nihilism. Nihilism would represent the “defeat” of the individual in the cold and dark universe we find ourselves in. Nihilism in existentialism is the recognition of a meaningless world but rather than accept this fact as fate, we labor against it in all the manifold ways we can: struggle for life; struggle for freedom; or giving a big middle-finger to the meaningless universe like Camus’ Sisyphus. Moreover, the phenomenon of existentialism as a purely “Western” philosophy is invariably linked to the West’s Christian heritage; the many themes of existentialism are nothing more than temporalized manifestations of themes already found in Christianity. Existentialism straddles the unique position of being metaphysically nihilistic but ontologically meaningful—if you happen to be an existentialist that is. If not then you might just see existentialism as exhausting in nihilism; returning to the very emptiness which it is grounded in.
 See William Barrett, Irrational Man: A Study in Existential Philosophy (1958).