4 Augustine replied: Since you continue repeating what has been so often exposed and refuted, we must be content to repeat the refutation. The things in the law and the prophets which Christians do not observe, are only the types of what they do observe. These types were figures of things to come, and are necessarily removed when the things themselves are fully revealed by Christ, that in this very removal the law and the prophets may be fulfilled. So it is written in the prophets that God would give a new covenant, "not as I gave to their fathers." Such was the hardness of heart of the people under the Old Testament, that many precepts were given to them, not so much because they were good, as because they suited the people.
Still, in all these things the future was foretold and prefigured, although the people did not understand the meaning of their own observances. After the manifest appearance of the things thus signified, we are not required to observe the types; but we read them to see their meaning. So, again, it is foretold in the prophets, "I will take away their stony heart, and will give them a heart of flesh," — that is, a sensible heart, instead of an insensible one. To this the apostle alludes in the words: "Not in tables of stone, but in the fleshy tables of the heart." The fleshy tables of the heart are the same as the heart of flesh.
Since, then, the removal of these observances is foretold, the law and the prophets could not have been fulfilled but by this removal. Now, however, the prediction is accomplished, and the fulfillment of the law and the prophets is found in what at first sight seems the very opposite.
Source: Reply to Faustus the Manichaean (New Advent)