It is a common assertion to state Christianity helped maintain the continuity of Classical civilization down to the Medieval era, through the “Dark Age” of Europe after the Fall of Rome. A more extreme position is that Christianity was a necessary condition for the maintenance of this civilizational tradition. I recall once reading an alternative history short story where illiterate tribesman visit the ruins of Rome, and muse about the consequences of Maxentius’ victory over Constantine at Milvian Bridge (this is the “point of departure”).
Obviously no one denies that the Christian Church was essential in maintaining ancient learning and ideas, whether through concrete steps such as copying in scriptoriums, or, more abstractly by integrating with into intellectual armamentarium tools developed by the Greeks (e.g., Greek philosophy). But, there is a line of thinking that asserts that there was something profound about the Christian religion which allowed for the maintenance of civilization against the barbarian hordes. Whether it is true or not is not an argument that is winnable in this space. But, the power of ideas to shape the course of human history is more tractable.
What I would suggest is that complex human phenomena, such as Christianity, are not reducible down to abstract sets of ideas in terms of how they manifest themselves in our world. That is, Christianity is only marginally about the Athanasian Creed, or even the sacrifice made by the Son of God, from a naturalistic perspective. Rather, the religion includes a broader set of institutions and folkways which derive from the culture at large (e.g., the Roman Catholic Church is the “ghost of the Roman Empire”). Additionally, it also expresses common human intuitions about the world and social relations.
But, as a complex cultural phenomenon, Christianity is conditional on complex culture. That is, Christianity may have aided the preservation of learning in the Dark Ages, but it couldn’t be the necessary cause of this preservation because too is an effect. The persistence of Christianity in the post-Roman world was a hallmark of those regions which maintained Romanitas to a greater extent. Christianity seems to have disappeared broadly (even if it persisted residually) from areas of the Roman Empire where there was total social collapse and transformation; the regions of Britain conquered by the Anglo-Saxons, much of the interior of Pannonia, Dacia, and Thrace. These are zones of cultural turnover. But, we know from genetics that a substantial local population persisted. In the Balkans and England a large minority of the ancestry derives from migrations which occurred after the year 500, but only a minority. But, the Roman majority clearly lost the cultural commanding heights, and with that the elite support for Christianity. These were zones that had to be re-Christianized in later centuries, even though a substantial proportion of the population probably had had Christian ancestors before.
It isn’t that there was a proactive campaign of paganization, analogous to what occurred in 17th century Japan against the Christian population, who were forced to register with Buddhist temples. Rather, the total defenestration of the old Roman elites in these areas made it so that the new elites seem to have had little incentive to convert and patronize the old religion. This is in contrast to the situation in post-Roman Gaul (Francia), Spain and Italy, where Roman era elites maintained enough continuity to influence the German warrior elites (though in many cases these elites were already Christian, they were Arian sectarians, whose religious difference marked them off from the old nobility and the peasantry).
This all came to mind when I began to read portions of Congo: The Epic History of a People. I am reading this book for two reasons. After Dancing in the Glory of Monsters: The Collapse of the Congo and the Great War of Africa, I have come to think that the Congo basin is one of the great laboratories of the forces which drive cultural geography. As such, I have an eye out for books on the Congo. Second, it was a summer reading deal for the Kindle, and so cheaper than a Starbucks coffee.
The second relevant to this post: after the decline of the Kingdom of Kongo a residual memory of Christianity persisted across broad areas. But, Christianity became integrated into African shamanism and folk religion, and lost all its substantive distinctiveness from African traditional religion. The few Europeans who ventured into the interior in the 19th century reported villages where there were survivals of Christian ideas, but they had transformed beyond simple recognition. In the 20th century the southwest portion Congo basin, which been under Kongo rule, therefore became the focal point for missionary activity again.
What is true for Christianity is probably true for many complex human ideas and institutions that we think are here for good. The reality is that complexity of thought and contingency of logic are dependent on the surpluses generated by a a highly developed economy and centralized state.
Addendum: The tendency to culturally evolve seems normal. It happened to Islam in China when it was isolated from the broader world Islamic community.