2 Augustine replied: How and for what purpose the Old Testament is received by the heirs of the New Testament has been already explained. But as the remarks of Faustus were then about the promises of the Old Testament, and now he speaks of the precepts, I reply that he displays ignorance of the difference between moral and symbolic precepts. For example, "You shall not covet" is a moral precept; "You shall circumcise every male on the eighth day" is a symbolic precept.
From not making this distinction, the Manichæans, and all who find fault with the writings of the Old Testament, not seeing that whatever observance God appointed for the former dispensation was a shadow of future things, because these observances are now discontinued, condemn them, though no doubt what is unsuitable now was perfectly suitable then as prefiguring the things now revealed. In this they contradict the apostle who says, "All these things happened to them for an example, and they were written for our learning, on whom the end of the world has come." The apostle here explains why these writings are to be received, and why it is no longer necessary to continue the symbolic observances.
For when he says, "They were written for our learning," he clearly shows that we should be very diligent in reading and in discovering the meaning of the Old Testament Scriptures, and that we should have great veneration for them, since it was for us that they were written. Again, when he says, "They are our examples," and "these things happened to them for an example," he shows that, now that the things themselves are clearly revealed, the observance of the actions by which these things were prefigured is no longer binding.
So he says elsewhere, "Let no man judge you in meat, or in drink, or in respect of an holy day, or of the new moon or of the sabbath-days, which are a shadow of things to come." Here also, when he says, "Let no one judge you" in these things, he shows that we are no longer bound to observe them. And when he says, "which are a shadow of things to come," he explains how these observances were binding at the time when the things fully disclosed to us were symbolized by these shadows of future things.
Source: Reply to Faustus the Manichaean (New Advent)