In The City of God (begun in 413, but Books 20-22 were written in 426) Augustine answers the pagans, who attributed the fall of Rome (410) to the abolition of pagan Divine Providence with regard to the Roman Empire, he widens the horizon still more and in a burst of genius he creates the philosophy of history, embracing as he does with a glance the destinies of the world grouped around the Christian religion, the only one which goes back to the beginning and leads humanity to its final term. The City of God is considered as the most important work of the great bishop. The other works chiefly interest theologians; but it, like the Confessions, belongs to general literature and appeals to every soul. The Confessions are theology which has been lived in the soul, and the history of God’s action on individuals, while The City of God is theology framed in the history of humanity, and explaining the action of God in the world.
Other apologetic writings, like the “De Verâ Religione” (a little masterpiece composed at Tagaste, 389-391), “De Utilitate Credendi” (391), “Liber de fide rerum quæ non videntur” (400), and the “Letter 120 to Consentius,” constitute Augustine the great theorist of the Faith, and of its relations to reason. “He is the first of the Fathers,” says Harnack (Dogmengeschichte, III, 97) “who felt the need of forcing his faith to reason.” And indeed he, who so repeatedly affirms that faith precedes the intelligent apprehension of the truths of revelation — he it is who marks out with greater clearness of definition and more precisely than anyone else the function of the reason in preceding and verifying the witness’s claim to credence, and in accompanying the mind’s act of adhesion. (Letter to Consentius, n. 3, 8, etc.) What would not have been the stupefaction of Augustine if anyone had told him that faithproofs of the divine testimony, under the penalty of its becoming science! Or if one had spoken to him of faith in authority giving its assent, without examining any motive which might prove the value of the testimony! It surely cannot be possible for the human mind to accept testimony without known motives for such acceptance, or, again, for any testimony, even when learnedly sifted out, to give the science — the inward view — of the object.
source: Catholic Encyclopedia