The secular realm is a Christian invention [which] goes back to the precept of Jesus to render unto Caesar what is Caesar’s, which later on was theorised by Augustine in his idea that human government will always be radically imperfect. The notion of separating religion from the practice of politics is a very wise precaution against fanaticism, which was, paradoxically but predictably, forgotten when secular moral enthusiasm replaced Christianity.
What then ensued were the political religions of the 20th century, which was the worst century in history in terms not only of the numbers murdered but also of the reason they were killed. People have always been killed by tyrants, and even by intolerant religions; but what was unique in the 20th century (though I think the first instances of it go back to the Jacobins) is that very large numbers of people were slaughtered in order to perfect humanity.
The notion of perfecting humanity by regenerative violence is new. And that is a mutation, it seems to me, of the moral passions of Western monotheism (in which I would include Islam in this respect) whilst removing the wise precautions that Augustine and other great Christian thinkers insist on, that, first, you cannot know exactly what the providential order is going to be in the world, so you should never take it on yourself to implement it, and, second, that human institutions are incorrigibly flawed, so you should never try to perfect them.
John Gray, Third Way, June 2004, Vol 27 No 5