There is a good deal that we simply cannot know about the history of the Roman Empire in the third and later centuries. To a greater or lesser extent, this is true of most periods of ancient history. Yet we must be careful to ask the questions we want to ask, rather than shifting towards those that the sources make it easiest to answer. In addition, the simple fact that so much Greek and Roman literature has failed to survive does actually suggest that the change from a Roman to medieval world was in many ways drastic. Far more of this literature was simply lost rather than deliberately suppressed or destroyed by churchmen. The medieval world was a far less literate place than the classical world that preceded it, particularly in western Europe. None of this suggests transformation. The fall of the Roman Empire was a major event, even if it occurred over considerable time and cannot be assigned to a specific date. 

From Adrian Goldsworthy’s The Fall of the West: The Slow Death of the Roman Superpower, London: Weidenfeld and Nicolson, 2009, p. 25.