What are the implications of believing there was a God who made the world or not?
Keller quotes Jean-Paul Sartre's landmark essay "Existentialism Is a Humanism" in his sermon "Before the Beginning", to draw out some enormous implications of believing there's a God that made us or not believing there's a God that made us. The following is Keller's paraphrase of excerpts from the essay:
If one considers an article of manufacture as, for example, a paper-knife - one sees that it has been made by a designer to serve a definite purpose. no one would produce a paper-knife without knowing what it was for. \Now if God created man, each human being is then the realization of certain Devine conception and purpose. Here, then, is the problem. The atheism of the 18th century abolished God, and yet still insisted that there was such a thing as human nature, that there were still some things that we must do because they are good things to do. Atheistic existentialism, of which I am a representative, declares with greater consistency that if God does not exist, we must face the consequences of this. We are not made for a purpose, like a paper-knife. And so therefore, there is no a priori good. Nowhere is it written that we must be honest and must not lie, because we are now on a plane in which there are only human beings and no God. As Dostoyevsky wrote, if there is no God, everything is permitted.
Read the (original) landmark essay here.