...if you were to destroy in mankind the belief in immortality, not only love but every living force maintaining the life of the world would at once be dried up. Moreover, nothing then would be immoral, everything would be lawful, even cannibalism. That's not all.... for every individual... who does not believe in God or immortality, the moral law of nature must immediately be changed into the exact contrary of the former religious law, and that egoism, even to crime, must become, not only lawful but... inevitable...rational, even honorable... (pgs 72 - 73)
Without religion, then, we see, in short order, the dismantling of love, and all morality. Indeed, the only possible good in such a bleak world would be self-interest pursued through whatever means necessary, including crime, even unto cannibalism if that floats one's boat. There can ultimately be no basis even for limiting the pursuit of animal pleasure to "not hurting anyone," for there can be no reason (save fear of punishment) for restraining oneself even from hurting others, if that is what gets you your jollies or works to your best advantage.
Of course, this is the exact line of reasoning used by Radion Romanovitch Raskolnikoff as a pretext for murdering the old pawn broker, Alena Ivanovna, in Dostoyevsky's Crime and Punishment. His best interest is to kill off the money lender to whom he owes a debt, knowing as he does that he can get away with and save himself the trouble of repaying the loan. The after effects of that fateful decision are enough to put the lie to the idea that there is no moral law for Raskolnikoff, thus showing the absurdity of such a position.
Sadly, however, our society seems deaf to these warnings. We've embraced irreligion as our religion and, as anyone with access to the internet can easily see, we are now beginning to witness the predicted consequences of public disbelief. More and more disordered and evil acts become first legal, then celebrated, then a right - we only need to think here of in utero infanticide or sodomy, divorce, and pornography - as we see first hand "the moral law of nature... immediately... changed into the exact contrary" of previous moral law. As we continue headlong down this alleyway, we'll soon enough get to the point where "every living force maintaining the life of the world (will) be dried up."
Luckily, societies, as well as individuals, can turn back; can repent and believe in the Gospel. That turning back, "conversion", starts, my friends, with you and me. May God give us the strength to do so, even against the prevailing trends of society.