6 Augustine replied: You understand neither the symbols of the law nor the acts of the prophets, because you do not know what holiness or righteousness means. We have repeatedly shown at great length, that the precepts and symbols of the Old Testament contained both what was to be fulfilled in obedience through the grace bestowed in the New Testament, and what was to be set aside as a proof of its having been fulfilled in the truth now made manifest. For in the love of God and of our neighbor is secured the accomplishment of the precepts of the law, while the accomplishment of its promises is shown in the abolition of circumcision, and of other typical observances formerly practised.
By the precept men were led, through a sense of guilt to desire salvation; by the promise they were led to find in the typical observances the assurance that the Saviour would come. The salvation desired was to be obtained through the grace bestowed on the appearance of the New Testament; and the fulfillment of the expectation rendered the types no longer necessary. The same law that was given by Moses became grace and truth in Jesus Christ. By the grace in the pardon of sin, the precept is kept in force in the case of those supported by divine help. By the truth the symbolic rites are set aside, that the promise might, in those who trust in the divine faithfulness, be brought to pass.
Source: Reply to Faustus the Manichaean (New Advent)