By the time of the Savior’s mortal ministry, the law of Moses had been at the foundation of Israelite religious and social life for over a thousand years. The Nephites possessed written records of the law on the brass plates, and Nephite prophets taught and observed the law. When the Savior visited the Nephites, He taught them that the law had been completely fulfilled in Him. However, they were not to think of the law of Moses as “destroyed” or having “passed away” (3 Nephi 12:17–18). How is it that the Savior “fulfilled” but did not “destroy” the law of Moses? The law of Moses included both moral and ritual aspects.
The moral aspects included such commandments as “Thou shalt not kill” and “Thou shalt not commit adultery.” Jesus Christ taught the Nephites that not only were they to avoid murder and adultery, but also anger and lust—conditions of the heart that lead to murder and adultery (see 3 Nephi 12:21–30). Thus the gospel of Jesus Christ fulfilled the law in the sense that it expanded the moral aspects of the law of Moses by being a higher law; it included the moral imperatives of the law of Moses and placed them in the context of broader gospel principles that require a change of heart.
The ritual aspects of the law of Moses included commandments about animal sacrifice and burnt offerings—what Abinadi called “performances” and “ordinances” (Mosiah 13:30). The Nephite prophets understood that these parts of the law of Moses were meant to help people look forward to the atoning sacrifice of Jesus Christ (see 2 Nephi 25:24; Jacob 4:5; Mosiah 16:14–15). Therefore, when the Savior’s mortal mission was completed, these forward-looking ordinances could no longer look ahead to a future event—the event had happened, and the ordinances were fulfilled in the sense that it concluded. Thus the Savior taught the Nephites that animal sacrifices and burnt offerings were to be “done away,” and that His followers were to offer instead the “sacrifice” of “a broken heart and a contrite spirit” (3 Nephi 9:19–20). In place of ordinances that looked forward to the Atonement, the Savior instituted the sacrament, an ordinance of remembrance, to look back to the Savior’s atoning sacrifice (see 3 Nephi 18:1–11).
Elder Bruce R. McConkie stated: “Jesus came to restore that gospel fulness which men had enjoyed before the day of Moses, before the time of the lesser order. Obviously he did not come to destroy what he himself had revealed to Moses anymore than a college professor destroys arithmetic by revealing the principles of integral calculus to his students. Jesus came to build on the foundation Moses laid. By restoring the fulness of the gospel he fulfilled the need for adherence to the terms and conditions of the preparatory gospel. No one any longer needed to walk by the light of the moon, for the sun had risen in all its splendor” (Doctrinal New Testament Commentary, 1:219–20; see also Stephen E. Robinson, “The Law after Christ,” Ensign, Sept. 1983, 68–73).