When I went to Oxford to study for a degree primarily about political history, we were told in our first week to read the Bible and given an exam on it at the end of Fresher's Week (while most people were making friends and getting drunk in Fresher's Week, Gracchi and his mates were sitting in the library learning Leviticus!) Christian politics is something that we all know- from Constantine to Benedict and Sarah Palin, the Church and its believers have sought to guide the state in its deliberations about what is and is not moral and what is and is not legal. Looking backwards the story of Christian interraction with politics is a fascinating one and ranges across a vast range of political possibilities, from the utopian radicalism of the Baptists in 16th Century Munster to the fierce reactionary spirit of Joseph de Maistre in 19th Century France and Russia. But there is also an equally strong tradition of Christian anti-politics: it has its roots in the Bible when Christ tells his disciples to render unto Caesar what is Caesar's, unto God what is God's.
Augustine's own views of Christian imperium are laid out in the 5th Book of the City of God. He describes the generic Christian Emperor thus:
We do not say that certain Christian Emperors were happy because they ruled for a longer time or because they died in peace and left sons behind to rule as emperors or because they subdued the enemies of the Commonwealth, or becaues they were able to avoid and supress uprisings against them by hostiel citizens. For even worshippers of demons... have deserved to receive these and other gifts and consolations of this wretched life.... Rather we say they are happy if they rule justly; if they are not lifted up by the talk of those who accord them sublime honours... but remember they are only men' (V 24)Augustine's model of Emperor is that of a saint- a man who did not have earthly gifts but who was 'slow to punish and swift to punish' (V 24), who exhibited private Christian moral qualities. This is key to the way that he views politics- politics is a place for exhibiting private virtues. As an activity it does not actually matter: Augustine says that for the security and morals of most men it matters little 'that some men should be conquered and others conquerors' (V 17)- it does not really effect who will go to heaven and hell. The whole purpose of life is eschatalogical and not political: and the purpose of rule is to demonstrate ethical values which are directed towards salvation not to ensure security or stability. God might intervene to help the Christian Emperor, as he did Augustine says help Theodosius (V 26) but Augustine also notes that 'God removed [Christian] Jovian far more quickly than he did [pagan] Julian' (V 25).
Augustine's view of politics is that it is secondary to the chief object of human life- salvation. Politics matters on a personal level to the Emperor but Augustine is unconvinced that much changes below the imperial level (perhaps this is a product of living in a pre-welfare state society). Even when eulogising Theodosius, he comments that
These deeds and similar ones which it would take too long to recall, are the good works that Theodosius bore with him from this temporal life where the greatest of human attainments and exaltation is but smoke. The reward of these works is eternal felicity which God gives only to those who are truly Godly. All the other things in this life, be they great or small, such as the world itself, light, air, earth, fruits, the soul and body of a man himself, sensation, mind, life; all these things he bestows upon good and evil men alike. And among these things is imperial sway also of whatever scope (V 26)Augustine offers no Christian prescription to keep a throne, makes no direct policy prescriptions save for be good and advance Christianity because ultimately these things do not matter. They are merely smoke. The thing that matters is salvation, politics is strictly secondary and may even by encouraging a lust for glory be an immoral activity. Whether the Diggers, Benedict, De Maistre and Palin agree, I'm not sure: but what this definitely represents is a relegation of politics to the second division of human concerns.