Augustine seeks to defend Christianity against various charges against it, especially those from paganism: that Rome had fallen to the Goths in 410 because Romans had abandoned its traditional gods by turning to Christianity.
To do so, Augustine explores the following themes: human piety and impiety, being just and unjust, and the city of God and the earthly city.
Augustine argued for the city of God under the authority of Christ. Even though Christ is its founder and king, this city’s existence can be traced from the beginning of human history, and will not exist in perfection until the end of history – history is therefore linear for Augustine.
Its members are those whose wills are oriented toward God and away from the self. It is a reality to be made known only eschatologically.
The earthly city is the opposite of the city of God, whose members’ wills are oriented toward self and away from God. This will lead not to loving concern for others but a lust for domination over them.
The earthly city is therefore characterised by struggle for power. It takes its visible form at least through the phenomenon of empire.
Yet Augustine welcomed the Roman Emperors being Christian. How can this be consistent?
Augustine defends the claim that the human good is not truly described in any previous philosophers’ conceptions because they only speak on earthly goods. The supreme human good is eternal life with God.
Augustine is particularly interested in the question of whether the human good is inherently social. He answers in the affirmative. However, the social life we live at present is far from the good life.
Peace, in its perfection, is the same as eternal life. He doesn’t mean minimal absence of conflict, but human wellbeing. Peace itself implicitly expresses his idea that the human good is social.
However chronically troubled earthly existence is, there is an inclination toward peace which evidences a longing for the good. Indeed, following the Platonic Socrates and the Stoics, a lack of good is a privation or distortion of being. So wherever there is any degree of peace evidences real good.
As such, what is of nature is of the original and good creation, before and apart from sin of which the advent was the event of the fall.
The principal reason for Augustine’s rejection of the naturalness of servitude is a conception of men as equal in a way which gives no ground for the subjection of some to others.
Rule by emperor and mastery over slaves are of the earthly city. The equality of men as all rational beings means that dominion over man is not the proper order of nature.
However, authority can be exercised if it is characterized by concern for the good of the other.
The peace of the city has three aspects: the member of the city of God will take his place in the activities necessary for maintaining it; as the Christian pilgrim participates in rule in the kingdom, he does so within an institution of fallen human existence; yet he will take part in this unavoidable exercise of authority for the peace of the city out of the same motivation as the head of the household does for its peace – with concern for the other’s good.
The peace of the earthly city (civil peace) is limited to a kind of compromise between human wills about the things relevant to mortal life – given members of the earthly city seek both imperium and proper peace. In the city of God, no limitations are necessary as there is no dominion.
In sum, both for members of the earthly city and for the city of God in its relation with the earthly city, civil peace is the indispensable modus vivendi.