Some hints ...
Basic "axiom" of L'existentialisme est un humanisme (1946) :
Atheistic existentialism, of which I am a representative, declares with greater consistency that if God does not exist there is at least one being whose existence comes before its essence, a being which exists before it can be defined by any conception of it. That being is man or, as Heidegger has it, the human reality. What do we mean by saying that existence precedes essence? We mean that man first of all exists, encounters himself, surges up in the world – and defines himself afterwards. If man as the existentialist sees him is not definable, it is because to begin with he is nothing. He will not be anything until later, and then he will be what he makes of himself. Thus, there is no human nature, because there is no God to have a conception of it. Man simply is. Not that he is simply what he conceives himself to be, but he is what he wills, and as he conceives himself after already existing – as he wills to be after that leap towards existence. Man is nothing else but that which he makes of himself. That is the first principle of existentialism.
For Sartre's slogan, see at least : “Existence Precedes Essence” and Jean-Paul Sartre :
the immense popularity of his scandalous public lecture “Existentialism is a Humanism,” delivered to an enthusiastic Parisian crowd October 28, 1945. Though taken as a quasi manifesto for the Existentialist movement, the transcript of this lecture was the only publication that Sartre openly regretted seeing in print. And yet it continues to be the major introduction to his philosophy for the general public. One of the reasons both for its popularity and for his discomfort is the clarity with which it exhibits the major tenets of existentialist thought [...].
The basis of Sartrean freedom is ontological: we are free because we are not a self (an in-itself) but a presence-to-self (the transcendence or “nihilation” of our self). [...] To that freedom [what Sartre calls “freedom as the definition of man” ]corresponds a coextensive responsibility. We are responsible for our “world” as the horizon of meaning in which we operate and thus for everything in it insofar as their meaning and value are assigned by virtue of our life-orienting fundamental “choice.”
Sartre sometimes talks as if any choice could be authentic so long as it is lived with a clear awareness of its contingency and responsibility. But his considered opinion excludes choices that oppress or consciously exploit others. In other words, authenticity is not entirely style; there is a general content and that content is freedom. Thus the “authentic Nazi” is explicitly disqualified as being oxymoronic. Sartre's thesis is that freedom is the implicit object of any choice, a claim he makes but does not adequately defend in his Humanism lecture. He seems to assume that “freedom” is the aspect under which any choice is made, its “formal object,” to revive an ancient term. But a stronger argument than that would be required to disqualify an “authentic” Nazi.
And see also Sartre's concept of Authenticity.
