In The City of God, St. Augustine points out that philosophers can very well establish a rational system of ethics, but we humans suffer from infirmities that prohibit us from consistently executing such a system on the grounds of rationality alone (Book II: 7).
It is tempting to say that this is why religion is needed: to socially enforce a system of ethics with norms, rituals, and rewards/punishments in the afterlife. But this is a trap, for it baits Christians into a Machiavellian ‘noble lie’ position, which is the diametric opposite of Christian ethics. This view would suggest that religion is only good because it is useful for some instrumental purpose. It would submit to Marx’s famous dictum about religion being an opium of the masses—or as we would say today, cope—and it would miss the much more compelling and radical insight of religious, and especially Christian, ethics.
There is a much more interesting riff on St. Augustine’s observation, but it requires a quick detour. Eventually we’ll arrive at the remarkable insight that religious ethics are more rational than rational ethics. But to get there, let’s start with how secular rationalists generally deal with St. Augustine’s critique.
Today, many intelligent individuals have grown skilled at backfitting a rational system of ethics to their own drives and appetites. In my view, this describes utilitarians like Sam Harris and many of the Effective Altruists. Polyamory, divorce, pornography—these types of things are perfectly justified because sex is pleasurable with no clear harm to others (so do as much as you want without harming anyone), marriage is just an agreement (so anyone can exit if it’s no longer maximizing utility), etc. We naturally want certain things, e.g. sex, so there’s nothing easier than adopting a system of ethics that says we should maximize the things that all people naturally want.
This is certainly one solution to St. Augustine’s dilemma! But it sneaks a faith-based or essentially religious assumption in the back door.
Utilitarians have to answer the question of why we are so lucky that unfettered personal preference is perfectly aligned with the Good. Isn’t it just a little too convenient? If it is truly the case that the correct system of ethics is so fully aligned with what intelligent people naturally desire, when there is no reason that this must be the case, then it is something of a miracle. Such a worldview therefore contains a God, in the sense that, for some cause or reason we do not understand, our personal drives and appetites are so profoundly aligned with social and cosmic order.
Utilitarians will argue that this alignment was generated by chance and evolutionary selection; millions of other potential civilizations have been generated by the universe's random number generator, but we've never heard about them because they were stillborn by the lack of this alignment. Ours seems like a miracle, but that's just an illusion caused by this selection effect.
Yet even here the rationalist is presuming the existence of an invisible lottery machine somehow operating since the beginning of time. It is not clear that this assumption is any more rational, or less faith-based, than the assumption of a personified creator God. Everyone has, at the end of their chain of reasoning, some agency or machinery that they posit at the beginning of time. In computer science, this would be called the universe's "kernel."
And now for my more traditional Christian friends, I should note that calling God the kernel of the universe is not any more sacrilege than for the Apostles to use the words "Counselor" and "Spirit of Truth," and many other labels, to refer to God. The fact they describe God at all is already a capitulation to the legitimacy of abstract computational formalization of the objective entity.
So utilitarian secular rationalism is arguably as faith-premised as Christian ethics, but perhaps it is superior because it is more consistent with what people really want.
High-IQ people who can consistently rationalize utilitarianism as the Good and not devolve to obviously atrocious behavior nonetheless face St. Augustine’s dilemma when it comes to lower-IQ people. Lower-IQ people are known to be less cooperative and more violent, on average. They are also more likely to indulge in a number of vices. They also have lower time preference, and cooperation mostly pays off in the long run, so it is often naively rational for lower-IQ people to steal, fight, etc. For this problem, utilitarians will say that we need rational governance, basically social engineering, which constrains and guides the masses toward optimal behaviors. From this viewpoint, devices ranging from minor paternalistic deceptions all the way to population-level eugenics are rational.
The result is a reign of Noble Lies. The lower-IQ masses will not organically abide by sophisticated arguments, and they cannot all be jailed or killed, so the secular rationalist is forced to commit, on the political plane, to Noble Lies. Basically whatever slogans or statements or media that are required to guide the lower-IQ masses into ethical behavior.
The problem is that once the Noble Lie reigns, middle-IQ people who may not be able to devise their own system of ethics will at least notice contradictions in the reigning system of legal and moral directives. Rightfully annoyed by an elite that presumes the mantel of rational social control while telling fibs, the midwit class will inevitably arrogate to itself the right to generate Noble Lies for their people. Why wouldn't they? Noble Lies are good and just according to the more knowledgeable and more powerful. Any purportedly logical system containing even one lie or contradiction can generate justifications for any statement whatsoever, and certainly the reigning Noble Lies will never maximize the interests of every single class equally. Therefore, ironically, it is under rationalism that epistemological chaos is most inevitable. What today we call "conspiracy theories" and "fake news" do not represent a decline of rationality in society. They are rather the necessary results of secular rationalism.
In the City of God, St. Augustine shows that the Christian system of ethics is the only system that, on average, generates advantageous results for the individuals and communities that adopt it, without introducing contradictions that corrupt Christian societies later. The price is that it requires the constant work of constraining our drives and appetites, periodic short-term persecution of believers, and explicit submission to a very particular theory about the universe's kernel.
From the perspective of our natural and automatic preferences, these attributes of the Christian system are sub-optimal indeed. We might be inclined to wish the universe was otherwise. But if the Christian system is correct in its theory of the kernel, then the advantage is that every layer of the human stack is logically consistent and each layer thrives in the long run.
When we refer to the long run, we are referring to the game-theoretic conception of the long-run equilibrium. The long run is the state of the world we expect to observe after infinite rounds of the game have been played. The words heaven, hell, and afterlife correctly capture that the long-run technically never arrives on Earth, although its mathematical reality is coherent, meaningful, and generative of predictive leverage.
Digital technology increases the speed with which human situations equilibrate toward their long-run. Hence the concept of accelerationism as well as the more mundane observation, noted by many, that life subjectively feels as though it moves faster nowadays.
The implication is that adopting allegiance to the correct belief-and-practice stack matters more with the onset of the digital revolution than before the digital revolution. Choose correctly and you spiral upward (the City of God), choose incorrectly and you spiral downward (the earthly City).
At this point, everyone must make their own wagers. Faith is no longer an option, the only question is where one decides to place one’s faith.