Faustus insists that the Old Testament promises are radically different from those of the New. Augustine admits a difference, but maintains that the moral precepts are the same in both.
1. Faustus said: Another reason for not receiving the Old Testament is, that both the Old and the New teach us not to covet what belongs to others. Everything in the Old Testament is of this kind. It promises riches, and plenty, and children, and children's children, and long life, and withal the land of Canaan; but only to the circumcised, the Sabbath observers, those offering sacrifices, and abstaining from swine's flesh. Now I, like every other Christian, pay no attention to these things, as being trifling and useless for the salvation of the soul. I conclude, therefore, that the promises do not belong to me. And mindful of the commandment, You shall not covet, I gladly leave to the Jews their own property, and content myself with the gospel, and with the bright inheritance of the kingdom of heaven. If a Jew were to claim part in the gospel, I should justly reproach him with claiming what he had no right to, because he does not obey its precepts. And a Jew might say the same to me if I professed to receive the Old Testament while I disregard its requirements.
2. Augustine replied: Faustus is not ashamed to repeat the same nonsense again and again. But it is tiresome to repeat the same answers, though it is to repeat truth. What Faustus says here has already been answered. But if a Jew asks me why I profess to believe the Old Testament while I do not observe its precepts, my reply is this: The moral precepts of the law are observed by Christians; the symbolic precepts were properly observed during the time that the things now revealed were prefigured. Accordingly, those observances, which I regard as no longer binding, I still look upon as a testimony, as I do also the carnal promises from which the Old Testament derives its name. For although the gospel teaches me to hope for eternal blessings, I also find a confirmation of the gospel in those things which "happened to them for an example, and were written for our admonition, on whom the ends of the world have come." So much for our answer to the Jews. And now we have something to say to the Manichæans.
3. By showing the way in which we regard the authority of the Old Testament we have answered the Jews, by whose question about our not observing the precepts Faustus thought we would be puzzled. But what answer can you give to the question, why you deceive simple-minded people by professing to believe in the New Testament, while you not only do not believe it, but assail it with all your force? It will be more difficult for you to answer this than it was for us to answer the Jews. We hold all that is written in the Old Testament to be true, and enjoined by God for suitable times. But in your inability to find a reason for not receiving what is written in the New Testament, you are obliged, as a last resource, to pretend that the passages are not genuine. This is the last gasp of a heretic in the clutches of truth; or rather it is the breath of corruption itself. Faustus, however, confesses that the Old Testament as well as the New teaches him not to covet. His own God could never have taught him this. For if this God did not covet what belonged to another, why did he construct new worlds in the region of darkness? Perhaps the race of darkness first coveted his kingdom. But this would be to imitate their bad example. Perhaps the kingdom of light was previously of small extent, and war was desirable in order to enlarge it by conquest. In that case, no doubt, there was covetousness, though the hostile race was allowed to begin the wars to justify the conquest. If there had been no such desire, there was no necessity to extend the kingdom beyond its old limits into the region of the conquered foe. If the Manichæans would only learn from these Scriptures the moral precepts, one of which is, Do not covet, instead of taking offense at the symbolic precept, they would acknowledge in meekness and candor that they suited the time then present. We do not covet what belongs to another, when we read in the Old Testament what "happened to them for examples, and was written for our admonition, on whom the ends of the world have come." It is surely not coveting when a man reads what is written for his benefit.
Source: Reply to Faustus the Manichaean (New Advent)