Leithart writes that Augustine saw apologetic significance of the fulfillment of Old Testament prophecies in the triumph of Christianity in the Roman Empire:
We might expect Augustine to launch into a detailed analysis of how the prophecies of the Old Testament were fulfilled in the gospel accounts, but he goes elsewhere. Psalm 2 prophesies that the son of David would rule the kings of the earth, and now “the name of Christ has spread through all the nations far and wide” and “the kings of the earth have themselves now become subject to the rule of Christ for their salvation and that all the nations are serving him,” just as the Psalm “predicted long before.” Augustine would show the pagan Psalm 72, and, citing passages from Jeremiah, would point to the fact that idols are everywhere overthrown. If the pagan were troubled by the unbelief of Jews, or the fact that the church is troubled by heresies, Augustine would point to prophecies that predicted precisely that….
Augustine is aware that arguments from Old Testament prophecy to the gospel story are vulnerable; the pagan could reply that the gospel writers fudged the life of Jesus to conform it to prophecy. But the fulfillments that Augustine highlights are within the immediate experience of the pagan interlocutor. And they all depend on taking Old Testament prophecies about the spread of the gospel quite literally. It is, we might say, a postmillennial proof from prophecy.
UPDATE (5/31/10): It appears that Leithart is drawing from Augustine’s Contra Faustum (Against Faustus). Here is another quote from the work that Leithart posted.