One of Western civilizations most important and well-respected piece of literature is The Brothers Karamazov by Feodor Dostoyevsky. Among other important religious issues, Dostoyevsky has one of his characters bring up the theme of permissiveness in society and its relationship to unbelief in a God.
Without God and immortal life? All things are permitted then, they can do what they like?
Or as Sartre commented:
Dostoevsky once wrote: "If God did not exist, everything would be permitted"; and that, for existentialism, is the starting point. Everything is indeed permitted if God does not exist, and man is in consequence forlorn, for he cannot find anything to depend upon either within or outside himself. He discovers forthwith, that he is without excuse.
In other words, Dostoyevsky implies that "Without a promise of ultimate reward, or fear of ultimate retribution, without God as the law-giver and sin-punisher, men are released from any motivation to observe their moral obligations, or even to justify those obligations in the first place." (Conifold, philosophy SE, #97098)
The results of this were seen and expressed by Dostoyevsky later:
If God did not exist, everything would be permitted; and that, for existentialism, is the starting point. Everything is indeed permitted if God does not exist, and man is in consequence forlorn, for he cannot find anything to depend upon either within or outside himself... Nor, on the other hand, if God does not exist, are we provided with any values or commands that could legislate our behavior. Thus we have neither behind us, nor before us in a luminous realm of values, any means of justification or excuse---We are left alone, without excuse. That is, what I mean when I say that, man is condemned to be free. (Quoted by Ibid)
So, does Dostoyevsky warn modern civilizations about the consequences of departing from Christianity? Or is too late; he is describing what modern civilization has already become? Condemned to be free! Or...?