According to Robert Matthews, that portion of the Savior's instruction that parallels the Sermon on the Mount seems to have a natural ending at the close of 3 Nephi 14, concluding with the parable of the houses built on sand and on rock. However, readers of the New Testament will remember that Matthew 7:28-29 adds a comment that Jesus' hearers in Palestine were "astonished at his doctrine: for he taught them as one having authority, and not as the scribes." The JST records this as: "He taught them as one having authority form God, and not as having authority from the Scribes." (JST Matthew 7:37.) At first glance it appears that this item concerning Jesus' authority is missing in the 3 Nephi account. Its parallel however, is in fact found in the subsequent chapter, in 3 Nephi 15:1-10. The two-verse, one-sentence observation of 28 words in Matthew is contrasted in 3 Nephi with a ten-verse, thirteen-sentence, 364-word declaration by Jesus. The 3 Nephi expression includes several concepts dealing with Jesus' authority. First Jesus affirmed that the instruction he had just given them contained "the things which I taught [in the Holy Land] before I ascended to my Father" (v. 1). He then explained: "I am he who gave the law" to Moses, and "the law in me is fulfilled" (vv. 3-8). He further established his identity, and thus his authority, by saying that he, Jesus, was the God who covenanted with Israel (v. 5). Earlier, at the beginning of this visit, he had declared to the multitude that he was "the God of Israel, and the God of the whole earth," and had "been slain for the sins of the world" (3 Nephi 11:14).
Jesus further attested to his authority by saying, "I am the law, and the light. Look unto me . . . and ye shall live," and he promised that unto those who obey him to the end, he would "give eternal life." (3 Nephi 15:9) The authority theme is continued with Jesus saying: "I have given unto you the commandments: therefore keep my commandments. And this is the law and the prophets, for they truly testified of me" (v. 10, emphasis added).
It would be difficult to miss the point that Jesus in these verses was giving an unqualified statement of his authority and primal position (under the Father) both on earth and in heaven. As noted, this is a much stronger statement than the brief comment by Matthew at the close of the sermon on the Mount, yet it is parallel to it in its placement at the end of the Sermon. [Robert J. Matthews, "Christ's Authority, His Other sheep, and the Redemption of Israel," in Studies in Scripture: Book of Mormon, Part 2, pp. 161-162]