“Existentialists, on the other hand, find it extremely disturbing that God no longer exists, for along with his disappearance goes the possibility of finding values in an unintelligible heaven. There could no longer be any a priori good, since there would be no infinite and perfect consciousness to conceive of it. Nowhere is it written that good exists, that we must be honest or not lie, since we are on a plane shared only by men.
Dostoevsky once wrote: ‘If God does not exist,everything is permissible.’ This is the starting point of existentialism. Indeed, everything is permissible if God does not exist, and man is consequently abandoned, for he cannot find anything to rely on – neither within nor without. First, he finds there are no excuses. For it is true that existence precedes essence, we can never explain our actions by reference to a given and immutable human nature.
In other words, there is no determinism – man is free, man is freedom. If, however, God does not exist, we will encounter no values or orders that can legitimize our conduct. Thus, we have neither behind us, nor before us, in the luminous realm of values, any means of justification or excuse.”
Jean-Paul Sartre, “Existentialism is a Humanism” (Yale University Press, 2007; p. 28-29)