Jean-Paul Sartre

Existentialism is a Humanism: here
Mantra: "existence comes before essence"

"Man is nothing else but that which he makes of himself. That is the first principle of existentialism."

"the first effect of existentialism is that it puts every man in possession of himself as he is, and places the entire responsibility for his existence squarely upon his own shoulders. 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."

"When we say that man chooses himself, we do mean that every one of us must choose himself; but by that we also mean that in choosing for himself he chooses for all men. For in effect, of all the actions a man may take in order to create himself as he wills to be, there is not one which is not creative, at the same time, of an image of man such as he believes he ought to be. To choose between this or that is at the same time to affirm the value of that which is chosen; for we are unable ever to choose the worse.What we choose is always the better; and nothing can be better for us unless it is better for all. If, moreover, existence precedes essence and we will to exist at the same time as we fashion our image, that image is valid for all and for the entire epoch in which we find ourselves. Our responsibility is thus much greater than we had supposed, for it concerns mankind as a whole... I am thus responsible for myself and for all men, and I am creating a certain image of man as I would have him to be. In fashioning myself I fashion man."

"Existentialism is not atheist in the sense that it would exhaust itself in demonstrations of the non-existence of God. It declares, rather, that even if God existed that would make no difference from its point of view. Not that we believe God does exist, but we think that the real problem is not that of His existence; what man needs is to find himself again and to understand that nothing can save him from himself, not even a valid proof of the existence of God. In this sense existentialism is optimistic. It is a doctrine of action, and it is only by self-deception, by confining their own despair with ours that Christians can describe us as without hope."