That order was challenged by the new, modernist varieties of rationalism. By the time Immanuel Kant wrote near the end of the eighteenth century, the content of rational theology was largely reduced to only what rational theologians imagined must be true of God in order to maintain the moral order (and thus civilization). Kant’s contribution on this front was to state the matter openly and honestly, and to provide a creative and formidable philosophical framework to give Enlightenment thinkers a place to stand.
In Kant’s proposal, theology is driven by the demands of practical reason, which governs the way we live in the world. The idea is rather simple: In order to live a moral life, one must believe certain things. One of those things is that doing what is right will lead to personal happiness. It is evident, however, that doing our moral duty does not always lead to personal happiness in this life; it often leads to suffering. Therefore, Kant argued that we must believe that there is some sort of afterlife in which the good is rewarded with happiness. Kant said that we cannot know that this state of affairs actually exists, but we find ourselves in the peculiar position of having to believe in such a state in order to do what reason demands is our duty. What is more, Kant concluded, we must also believe that what reason requires of us is backed by divine authority and thus what God commands, just as we must also believe that God will reward those who do their duty in this life with endless happiness in the next.
In some ways, Kant’s framework completed the restructuring of ethics from the previous view that prevailed in the West; in other ways, it accelerated the unraveling of the broad moral consensus that earlier view supported. Previously, ethics was often approached as a branch of theology; after Kant, ethics has been generally approached as an autonomous discipline, as it had once been conceived of in Athens. So long as Westerners continued to think more or less like Christians on moral matters and to endorse the main contours of Christian moral thought as revealed in Scripture and summarized in the Decalogue, this seemed reasonable. But the assumption that human notions of morality were stable proved naive. Once the theological guy-wires were cut, ethics was swept up in the whirlwind.
Subsequent thinkers would propose metaethical principles derived from anthropology, sociology, psychology, and eventually even biology. Meanwhile, suspicious critiques of the gospel voiced in the eighteenth century (e.g., by Hermann Reimarus) were further developed and extended to Christian moral teaching by nineteenth-century thinkers such as Karl Marx, Friedrich Nietzsche, and Sigmund Freud. The particulars of their critiques varied widely, but these “masters of suspicion,” as Paul Ricoeur calls them, each proposed counternarratives of Christian morality that cast it as a deviant or debilitating historical development and an impediment to personal and social progress.
As harsh and unhinged from historical reality as these counternarratives sometimes were, suspicion toward Christianity is all one has left when the possibility of revealed truth is rejected. The fact that Christianity exists as a potent force in the world and has the structure and form that it does demands an explanation. But if Christian theology is not true, the reasoning runs, then Christian teaching and practice must serve some other purpose than the purpose it appears or pretends to serve. Some suspect Christianity of being a coping mechanism for despair, fear, unfulfilled desires, or suffering; others suspect it of being an instrument of oppression that allows adherents to restrain and impose their collective will on others; yet others suspect moral discourse is meaningless or that morality itself is an evolutionary illusion.