What is the church to say to Caesar when he converts? This was not something directly pondered in the New Testament, but thinking about it has caused quite a headache ever since Constantine decided to throw his lot in with the all-conquering Galilean. Augustine offered one of the most influentual accounts in the following passage:
We say that they are happy if they rule justly; if they are not lifted by the talk of those who accord them sublime honours or pray their respects with an excessive humility, but remember that they are only men; if they make their power the handmaid of His majesty by using it to spread His worship to the greatest possible extent; if they fear, love and worship God; if they love that Kingdom which they are not afraid to share with others more than their own; if they are slow to punish and swift to pardon; if they resort to punishment only when it is necessary to the government and defence of the commonwealth, and never to gratify their own enmity; if they grant pardon, not so that unjust men may enjoy impunity, but in the hope of bringing about their correction; if they compensate for whatever severe measures they may be forced to decree with the gentleness of mercy and the generosity of benevolence; if their own self-indulgence is as much restrained as it might have been unchecked; if they prefer to govern wicked desires more than any people whatsoever; if they do all these things not out of craving for empty glory, but from love of eternal felicity; and if, for their sins, they do not neglect to offer to their true God the sacrifices of humility and contrition and prayer. We say that, for the time being, such Christian emperors are happy in hope and that, in time to come, when that to which we now look forward as has arrived, they will be so in possession.
- Augustine, The City of God book 5, chapter 24.UPDATE: For a very accessible summary of contemporary scholarship on City of God that happily confounds reading Augustine as either a secular liberal before his time or a dastardly apologist for Christendom, check out this lecture. He includes a lengthy summary of the elusive Rowan Williams article that has been mentioned in the comments.