When we speak of forlornness, a term Heidegger was fond of, we mean only that God does not exist and that we have to face all the consequences of this. The existentialist is strongly opposed to a certain kind of secular ethics which would like to abolish God with the least possible expense. About 1880, some French teachers tried to set up a secular ethics which went something like this: God is a useless and costly hypothesis; we are discarding it; but, meanwhile, in order for there to be an ethics, a society, a civilization, it is essential that certain values be taken seriously and that they be considered as having an a priori existence. It must be obligatory, a priori, to be honest, not to lie, not to beat your wife, to have children, etc., etc. So we’re going to try a little device which will make it possible to show that values exist all the same, inscribed in a heaven of ideas, though otherwise God does not exist. In other words — and this, I believe, is the tendency of everything called reformism in France — nothing will be changed if God does not exist. We shall find ourselves with the same norms of honesty, progress, and humanism, and we shall have made of God an outdated hypothesis which will peacefully die off by itself.
The existentialist, on the contrary, thinks it very distressing that God does not exist, because all possibility of finding values in a heaven of ideas disappears along with Him; there can no longer be an a priori Good, since there is no infinite and perfect consciousness to think it. Nowhere is it written that the Good exists, that we must be honest, that we must not lie; because the fact is we are on a plane where there are only men. Dostoievsky said, “If God didn’t exist, everything would be possible.” That is the very starting point of existentialism. Indeed, everything is permissible if God does not exist, and as a result man is forlorn, because neither within him nor without does he find anything to cling to.
Jean-Paul Sartre, “Existentialism is a Humanism”