For his thematic image in the City of God, Augustine expanded upon the philosophical ideas included in the Republic by Plato, and used the theological idea of the kingdom of God from the New Testament.
And for his world view Augustine took the opportunity to detail an ideal of a new universal Christian world order that would mirror the perfection of the kingdom in heaven.
In contrasting the earthly and heavenly cities that represent the continuous struggle between good and evil, the City of God explores human history in its relation to all eternity.
For a thousand years it was for Europe a guide to the rights and duties of the individual vis a vis the state.
It showed how a person was subject simultaneously to his temporal lord and to the spiritual kingdom which was the Church.
The City of God is the longest single work presenting a sustained argument unified around a coherent single theme to survive from Greek or Roman antiquity.
It is also by far the lengthiest single work that Augustine ever wrote, and far longer than any of the surviving works of Plato or Cicero. Even Augustine himself thought that City of God was a bit long!
To embrace these numerous areas of learning in the City of God, Augustine inter-related aspects of philosophy, theology, ecclesiology, politics and anthropology at a time in Western history when these areas of learning essentially did not yet exist as separate disciplines.
Peter Brown, a contemporary author on Augustine, has stated that City of God was not a static final statement by Augustine, but his sequential and often-interrupted point-by-point attacks on as much of the pagan literature that was available to him. The book was not formal political theory, but treatede what theologically the Church terms as the Communion of Saints.
And then it would require from Augustine the exposition of a world view consistently throughout all that he wrote. This appeared to be a lifelong task - as in one sense it became.
Augustine accepted this considerable challenge and, more remarkably again, completed the task even though he had time only to work on it occasionally over thirteen years, and thus had to publish portions of it progressively.
The City of God became his major work, and one in which the required breadth of vision suited his encyclopedic mind and encyclopedic memory.