St. Augustine of Hippo, The City of God (426)

“[Christian emperors] are happy if they rule justly; if they are not lifted up amid the praises of those who pay them sublime honors, and the obsequiousness of those who salute them with an excessive humility, but remember that they are men; if they make their power the handmaid of His majesty by using it for the greatest possible extension of His worship; if they fear, love, worship God; if more than their own they love that kingdom in which they are not afraid to have partners; if they are slow to punish, ready to pardon; if they apply that punishment as necessary to government and defense of the republic, and not in order to gratify their own enmity; if they grant pardon, not that iniquity may go unpunished, but with the hope that the transgressor may amend his ways; if they compensate with the lenity of mercy and the liberality of benevolence for whatever severity they may be compelled to decree; if their luxury is as much restrained as it might have been unrestrained; if they prefer to govern depraved desires rather than any nation whatever; and if they do all these things, not through ardent desire of empty glory, but through love of eternal felicity, not neglecting to offer to the true God, who is their God, for their sins, the sacrifices of humility, contrition, and prayer.”

“The kingdom of the world has become the kingdom of our Lord and of his Christ, and he shall reign forever and ever.”
—Revelation 11:15