Along with the scriptures, the study of St. Augustine and St. Aquinas remains essential for Catholics to understand the founding stones of their faith.
The major work of St. Augustine, The City of God, lays out the ground for us to understand the world (city of man) through an understanding of the divine, and the city within which the faithful and just live, (City of God).
For many generations it was misunderstood as a rationale for temporal rule by the Church, but that has been corrected, as this article from 30 Days notes.
“It is interesting to note that Augustine’s present relevance coincides with the unfashionableness of the medieval version of his thought, with the definitive waning of that political Augustinism which supplied the theoretic justification of papal supremacy over imperial power in the running quarrel that lasted from Gregory VII until Boniface VIII. The many studies that have emerged in recent decades on the work of the bishop of Hippo, from that of Reinhold Niebuhr to those of Étienne Gilson, Sergio Cotta, Joseph Ratzinger and others, all proceed from a re-appraisal of Augustine’s position, in particular that expressed in De civitate Dei, [The City of God] along with a critique of medieval political Augustinism. The theses of these studies could be summed up as follows: for Augustine the dualism between the two civitates, the “city of God” and the “earthly city” is not to be identified with the conflict between Church and State. “The City of God, with resplendent walls of adamantine, is the supernatural goal of the believer; with Saint Augustine, it becomes achievable in this life. All just men are citizens. The conflict between Christians and Romans, Church and Empire, provincials and government ceases: it shifts into the conscience”. Augustine’s model, secondly, differs both from the potentially revolutionary eschatology of Origen – which tends to de-legitimize the ordinances and laws of the State because they are not in line with the requirements of the Gospel – and from the political theocracy of Eusebius of Caesarea who, identifying Christian universalism with that of Rome, laid the ideological basis on which Byzantium was to found its “Christian” empire. This double distinction, from Origen and from Eusebius, enables us, in third place, to see the model set out in De civitate Dei as absolutely not theocratic, and this despite the fact that during the Donatist controversy, particularly in his Letter 93 to Bishop Vincentius, Augustine lets one glimpse a possible use in that direction. It is this “use” that explains the “political Augustinism” which, as Gilson well explains, led his followers to affirm “a dual and complementary tendency. On the one hand, forgetful of the great apocalyptic vision of the heavenly Jerusalem, they reduced the City of God to the Church, which in Augustine’s authentic view was no more than the ‘pilgrim’ part, working in time to enlist citizens for eternity. On the other, there was the ever growing tendency to mistake the earthly city of Augustine – the mystical city of perdition – with the temporal and political city. From then on the problem of the two cities became that of the two powers, the spiritual power of the popes and the temporal one of States or princes”.