Stace, who was himself a non-theist, put it this way:
The real turning point between the medieval age of faith and the modern age of unfaith came when scientists of the seventeenth century turned their backs upon what used to be called "final causes"... [belief in which] was not the invention of Christianity [but] was basic to the whole of Western civilization, whether in the ancient pagan world or in Christendom, from the time of Socrates to the rise of science in the seventeenth century.... They did this on the ground that inquiry into purposes is useless for what science aims at: namely, the prediction and control of events.Stace is correct. If there is no God then life is a pointless absurdity and good and evil have no objective referent. An atheist, to be consistent, should embrace existential nihilism and give up lecturing the rest of us about right and wrong. Or, if he wishes to retain his belief that life is purposeful and that cruelty, slavery, ecological destruction, indifference to the plight of the poor, etc. are objectively wrong, he should repudiate his atheism. Atheism simply offers no grounds whatsoever for making any moral judgment.
...The conception of purpose in the world was ignored and frowned upon. This, though silent and almost unnoticed, was the greatest revolution in human history, far outweighing in importance any of the political revolutions whose thunder has reverberated around the world....
The world, according to this new picture, is purposeless, senseless, meaningless. Nature is nothing but matter in motion. The motions of matter are governed, not by any purpose, but by blind forces and laws....[But] if the scheme of things is purposeless and meaningless, then the life of man is purposeless and meaningless too. Everything is futile, all effort is in the end worthless.
A man may, of course, still pursue disconnected ends, money, fame, art, science, and may gain pleasure from them. But his life is hollow at the center. Hence, the dissatisfied, disillusioned, restless spirit of modern man....Along with the ruin of the religious vision there went the ruin of moral principles and indeed of all values....
If our moral rules do not proceed from something outside us in the nature of the universe - whether we say it is God or simply the universe itself - then they must be our own inventions. Thus it came to be believed that moral rules must be merely an expression of our own likes and dislikes. But likes and dislikes are notoriously variable. What pleases one man, people, or culture, displeases another. Therefore, morals are wholly relative.
To the extent that the non-theist seeks to have it both ways - which, of course, most of them do - he's acting irrationally.