18 The Manichæans, therefore have no ground for saying, in disparagement of the law and the prophets, that Christ came to destroy rather than to fulfill them, because Christians do not observe what is there enjoined: for the only things which they do not observe are those that prefigured Christ, and these are not observed because their fulfillment is in Christ, and what is fulfilled is no longer prefigured; the typical observances having properly come to a close in the time of those who, after being trained in such things, had come to believe in Christ as their fulfillment.
Do not Christians observe the precept of Scripture "Hear, O Israel; the Lord your God is one God;" "You shall not make unto you an image," and so on? Do make Christians not observe the precept, "You shall not take the name of the Lord your God in vain?" Do Christians not observe the Sabbath, even in the sense of a true rest? Do Christians not honor their parents, according to the commandment? Do Christians not abstain from fornication, and murder, and theft, and false witness, from coveting their neighbor's wife, and from coveting his property,— all of which things are written in the law?
These moral precepts are distinct from typical sacraments: the former are fulfilled by the aid of divine grace, the latter by the accomplishment of what they promise. Both are fulfilled in Christ, who has ever been the bestower of this grace, which is also now revealed in Him, and who now makes manifest the accomplishment of what He in former times promised; for "the law was given by Moses, but grace and truth came by Jesus Christ." Again, these things which concern the keeping of a good conscience are fulfilled in the faith which works by love; while types of the future pass away when they are accomplished. But even the types are not destroyed, but fulfilled; for Christ, in bringing to light what the types signified, does not prove them vain or illusory.
Source: Reply to Faustus the Manichaean (New Advent)