Among the Church Fathers it is Augustine in particular who asserted Christianity in a scientific philosophical presentation over against classical philosophy, he is the primary founder of Christian speculation, and he became decisive for the entire Middle Ages in science and culture [Lebensgestaltung].
The basic idea of his book Of the City of God (de civitate Dei) is the antithesis of a heavenly state (civitas Dei, civitas coelestis) and an earthly state (civitas terrena), founded on the biblical contrast of God’s children and the children of the world; but in a speculative development. To wit, it conceives this opposition as a world-historical economy rather than as the mere nature of individual human beings, it is the contrast between two coherent organized areas of sentiments, aspirations, tasks, but also institutions. Destinies, world designs that run right through earthly life. It is, if one wishes to use a more modern term, a construction of world history. He begins the demonstration of this opposition at Cain and Abel, and conducts it on the one hand through secular history, the empires of the Assyrians, Persians, Greeks, Romans, and on the other hand through the sacred history of the Patriarchs, the Jewish judges, kings, prophets, up until the appearing of Christ and the Christian church. The end is the eternal glorification of God-state and the downfall and the destruction of the earthly state.
The term: state (civitas) is used symbolically in both relationship, and therefore the question is, how does the real state on earth, the civic organization, behave toward these two states? The expressions of Augustine on this score are not clear, in details perhaps even contradictory; nevertheless, the thoughts that form his total conception can be identified with certainty.
The civil state is it not the same to him as the “earthly state” (civitas terrena), although he alternately uses the term civitas terrena for the realm of secular, and for the state. Because he recognizes that the purpose of the state, earthly peace (as opposed to eternal peace), in itself is not blameworthy, and also that Christians should wish for that peace and make use of it as long as mortality continues, that precisely for this reason they also owe the state obedience and to this end should have community with the earthly-minded. Accordingly, it cannot be something absolutely evil, as is worldly-mindedness, and that which is spoken of regarding the “earthly city” cannot refer to him, that with the devil it will fall under eternal judgment.
But yet he brings the state into the closest connection with the earthly kingdoms. For him the state, and not merely the current condition of the state, but the existence of a state generally, is simply the consequence of sin, and therefore also a mere emergency institution, an arena of the passions and the oppression of the weak by the powerful. Accordingly, he considers interest in the state as something purely secular. Therefore the establishment of states everywhere proceeds from the earthly-minded; the heavenly-minded do not worry about it. In this manner, the first state was founded by Cain, the original representatives of the earthly kingdom – (Cain built a city) – while Abel was a stranger on earth and an exile. Likewise, the state was κατ’ ἐξοχήν, the Roman, founded by Romulus, who, earthly-minded, also murdered his brother, and is a type of Cain. While the classical view considers the state to be the highest in human life, for Augustine it occupied the extreme opposite position, as something deeply subordinate for the true man. This is not to ignore how much the practice or allowance of pagan worship, which Augustine experienced in reality, was determinative of his judgment.
Nonetheless, he energetically asserts that only Divine Providence determines the formation of states and their fates, and he even takes it upon himself partly to show the moral motivations of this Providence in the governance of world destinies. In particular, he conducts a comprehensive discussion of how the Romans were allotted the empire by God for the sake of their civic virtues.
In accordance with such an assessment of the state, Augustine pays scant attention to the actual political tasks of the same and puts its main function only in the protection of the church. That and only that appears to him as the truly ethical significance of the state, understandably because in this way it enters into the service of the celestial empire (civitas coelestis). The practical goal in Augustine’s doctrine of the state is therefore the maintenance of the ecclesiastical rules and the restriction and punishment of heretics by the external power of the authorities; that is the idea which most vividly fills Augustine and by which he became so decisive to the subsequent shape of Christianity.
Running through this whole conceptualization, the true insight is given with the Christian revelation, that the state, like the entire human condition, is estranged from its true eternal form, that an act changed the original condition of human existence, and that an act will one day again restore it. This insight was lacking to the Greeks; it is the middle term which alone can unite Plato and Aristotle. From it, the contradiction between the absolute moral task of the human race, of which Plato is filled, and the laws of nature and the natural constitution of man, which Aristotle makes into a standard, is solved. Supported by it, one neither needs, as Plato does, to require an arrangement of human coexistence, of which man is incapable, for the sake of their ethical truth, nor does one, like Aristotle, because of this human incapacity, need to tone down the moral requirement on the individual and the whole, and to hold the mere (negative) midpoint between two vices to be the nature of virtue, instead of recognizing the positive and infinite content thereof. Proceeding from it, the absolute devotion of the people, which Plato requires for the state, is referred to another realm than this ephemeral and tenuous one. Herein Augustine, by virtue of Christian enlightenment, extended moral and legal philosophical knowledge infinitely beyond classical wisdom. He gave it its true center. But Augustine fails to recognize the independent and divine significance that earthly living conditions and thereby above all the state still have, even in their present turbidity. He flees from the secular sphere, rather than penetrate it with the recovered truth. The idea of the state as state, heroism, justice, wisdom, like the arrangements and tasks of well-ordered, properly structured public life lie beyond his attention, are not in themselves divine and worthy of Christian interest, and the significance which he concedes to one of the two great institutions of God, the state, is not that it solve its own task, but only that it come to the aid of the other, the church, for its tasks. Basically then for Christianity only the church can remain, and this makes use of the state to realize its rules externally – thus forcibly. Therein lies the origin of the hierarchical idea, according to which the church erects itself as a state, as a theocratic empire, and maintains the faith as a civil law, so that heresy, as the supreme civil crime, commands the death penalty. The state, as Augustine conceives it, is either completely unnecessary and worthless for Christianity, or the church must truly and fully use it. This is the result when the state is denied its own importance, and the protection of the Church is recognized as its sole task. By contrast, supposing the independent importance of the state, then its connection with the church, which Augustine uttered, is an indestructible truth, which has been tested from that hour to this among the Christian nations, and is being challenged at this time only because the peoples are no longer imbued with the same vitality of the Christian faith. Because the separation of church and state, which is now a solution, is only taught by those who either share this lack of faith, or cannot visualize a different public condition than this one which surrounds them.
 City of God, Bk. XV ch. 4, Bk. XIX chs. 14, 16, 17.
 “Aeternum supplicium subire cum diabolo,” Bk. XV, chs. 1 and 4.
 Bk. XIX ch. 15.
 Bk. XV ch. 1.
 Cf. Bk. XI ch. 1 and Bk. XIX ch. 21.
 Bk. IV ch. 31.
 Bk. V ch. 15.
 This notion is especially realized in a few of his letters: cf. epist. 93 ad Vincent., epist. 185 ad Bonifac.