Jean-Paul Sartre, answered from the texts and cited to the page.
The paper knife is the clearest way in. A craftsman makes it with a purpose already in mind: the concept — what it is for, how it will be shaped — exists before the object itself is hammered into being.1 God, on the traditional account, relates to man the same way: each human being is the realization of a concept within the divine intelligence, a particular instantiation of a universal human nature.2
Remove God, and you might expect that to change everything. The eighteenth-century atheists — Diderot, Voltaire, even Kant — did not let it change anything; they kept the human nature, the universal concept of man, and simply dropped the divine craftsman who had supposedly conceived it.3 Atheistic existentialism is more consistent than that.
If there is no God to have conceived man in advance, then man arrives in the world before any definition of him is possible. He materializes, encounters himself, and only afterward defines himself.4 He is, to begin with, nothing — not a degraded nothing, but an open nothing, a nothing that is the very condition of his being able to become something at all.
What he becomes is what he makes of himself, and nothing else.5 This is not merely a thesis about chronological sequence. It is an ontological claim about the structure of human existence as such. Freedom is not a quality added onto a pre-given human nature; it is, as I put it in Being and Nothingness, the very fabric of my being.6 My existence cannot be contained within a definition precisely because it is freedom that is the foundation of all essences — man discloses the essences of things in the world by surpassing the world toward his own possibilities, and he cannot do that surpassing if he is himself already an essence, already fixed.7
What follows from this is responsibility, and responsibility of a vertiginous scope. Since man is nothing other than what he makes of himself, he is responsible for what he is — not only for his own individuality but, in a sense I developed at length in the 1945 lecture, for all men.8 When I choose, I choose a certain image of man; I cannot choose otherwise than to choose for everyone.
That is the weight existence-before-essence places on every act. There is one further complication worth naming. In Truth and Existence I pressed the point that each truth — each disclosure of being — is irreparable, dated, historical, and mortgages the infinity of the future.9 Consciousness reveals being in order to exist, and in revealing being it makes the revealed irrevocably "having been." Freedom, then, is not the airy freedom of a subject who floats above the world choosing costumes.10
It is the freedom of a being who is always already thrown into a situation it did not create, who must assume responsibility for what it has neither created nor wanted, and who cannot escape from that condition.11 Existence precedes essence means: you arrived first, unfinished, and you are accountable for what you do with that.
if we consider a manufactured object, such as a book or a paper knife, we note that this object was produced by a craftsman who drew his inspiration from a concept: he referred both to the concept of what a paper knife is, and to a known production technique that is a part of that concept.Existentialism Is a Humanism, pp. 35–36
Thus each individual man is the realization of a certain concept within the divine intelligence.Existentialism Is a Humanism, pp. 35–36
Eighteenth-century atheistic philosophers suppressed the idea of God, but not, for all that, the idea that essence precedes existence. We encounter this idea nearly everywhere: in the works of Diderot, Voltaire, and even Kant.Existentialism Is a Humanism, pp. 35–36
man first exists: he materializes in the world, encounters himself, and only afterward defines himself.Existentialism Is a Humanism, pp. 36–38
man is nothing other than what he makes of himself. This is the first principle of existentialism.Existentialism Is a Humanism, pp. 36–38
it is not a quality that is added on, or a property of my nature; it is, to be exact, the fabric of my being.Being and Nothingness, pp. 586–587
it is freedom that is the foundation of all essences, since it is by surpassing the world toward his own possibilities that man discloses intraworldly essences.Being and Nothingness, pp. 586–587
man is responsible for what he is. Thus, the first effect of existentialism is to make every man conscious of what he is, and to make him solely responsible for his own existence. And when we say that man is responsible for himself, we do not mean that he is responsible only for his own individuality, but that he is responsible for all men.Existentialism Is a Humanism, pp. 36–38
Each truth is both dated and historical, and it mortgages the infinity of the future.Truth and Existence, pp. 95–96
consciousness cannot choose an end without at the same time choosing the truth, it cannot make itself be without making itself be before Being, that is, without revealing.Truth and Existence, pp. 95–96
I have shown that freedom always means assuming our responsibilities afterwards for what we have neither created nor wanted... But it cannot escape from its condition.Truth and Existence, pp. 95–96