From Hilaire Belloc (1870–1953), Anglo-French writer, historian, orator, poet, soldier and political activist, attributed in A Year to Live: How to Live This Year as If It Were Your Last (1998), by Stephen Levine:
The Barbarian hopes — and that is the mark of him, that he can have his cake and eat it too. He will consume what civilization has slowly produced after generations of selection and effort, but he will not be at pains to replace such goods, nor indeed has he a comprehension of the virtue that has brought them into being. Discipline seems to him irrational, on which account he is ever marvelling that civilization, should have offended him with priests and soldiers…. In a word, the Barbarian is discoverable everywhere in this, that he cannot make: that he can befog and destroy but that he cannot sustain; and of every Barbarian in the decline or peril of every civilization exactly that has been true.
We sit by and watch the barbarian. We tolerate him in the long stretches of peace, we are not afraid. We are tickled by his irreverence; his comic inversion of our old certitudes and our fixed creed refreshes us; we laugh. But as we laugh we are watched by large and awful faces from beyond, and on these faces there are no smiles.
Here’s the problem: The discipline of the old certitudes and fixed creed – as defined and imposed by priests (religious and secular) and soldiers – is irrational, and has no more comprehension of the virtue that brings the goods of civilization into being than the barbarian does. Ayn Rand’s article Faith and Force: The Destroyers of the Modern World is a good brief discussion; it can be found by searching on the title. For an explanation in depth, particularly as regards Fred Reed’s Whither the Shards of America?: It’s Us, or Them see The Ominous Parallels by Leonard Peikoff.