I like it when I run across a historian who supports Belloc's popular histories. For kicks, I recently started reading Simon Schama's A History of Britain. On page 45, he writes about the transition from Roman Britain to Anglo-Saxon Britain:
A tidy compartmentalization of British history, with the wholesale destruction of Roman Britain immediately followed by its violent reincarnation as Anglo-Saxon England, bears no relation to the experience of the vast majority of the inhabitants of Britain. The governing institutions of the Roman province did, indeed, fall away, but much of the social practices and culture and even the language of the old Britain persisted long after the arrival of the first few bands of Saxon mercenaries and freebooters. For many generations Romano-Britons and North Sea warriors must have lived alongside each other, as neighbours rather than implacable foes.
This parallels Belloc's discussion of the "fall" of the Roman Empire in Europe and the Faith. Now, in England, the withdrawal of the last Roman legions really was a "fall," so it's not quite the same thing that Belloc wrote about, but it's similar, in that the withdrawal of the last legions didn't result in a huge cultural/societal change. Things pretty much kept doing as always, with a slow decline. Britain was still Roman after the Romans left. Similarly, Europe remained "Roman" for many years after the Vandals sacked Rome in 454, except on the continent, the Roman "leftovers" were much more pronounced because the people and their rules still saw themselves as Roman subjects, albeit governed from Constantinople, from which little or nothing was heard.
