3 Nephi 20:16–19 quotes from Micah 5:8–9 and 4:12–13, especially to show that all “gain” would be consecrated unto the Lord (20:19). What Micah (and Isaiah) meant by “the remnant of Jacob” (Micah 5:8; 3 Nephi 20:16 see also Isaiah 10:21–22) remains obscure, but Jesus quotes Micah here after saying that there will be many “remnants” scattered abroad (20:13–14). One remnant of Israel (or Jacob) in particular “will return” unto the Lord and it will be a great sign when “these things” will come forth “unto them” (21:4).
Then, in 3 Nephi 20:22–25, Jesus went on to teach about the covenant that had been made with Jacob regarding the land (20:22) and also the covenant made with Abraham (20:25, 27) regarding the blessing of all the kindreds of the earth through his seed. It is interesting that he lists these two patriarchs in the reverse order from our normal expectations. We often talk about the covenant of Abraham, Isaac and Jacob, but here it is first the covenant of Jacob, and then Abraham. In Leviticus 26:42, the Holy of Holies of the book of Leviticus, the Lord likewise promises, in reverse order, “Then will I remember my covenant with Jacob, and also my covenant with Isaac, and also my covenant with Abraham will I remember.” I think we have here in the Book of Mormon a credible and appropriate reference here back to the holiest spot of the book of Leviticus, which is the holiest of all places in the Old Testament.
Both there and here, God promises many things, but above all he promises that he will fulfill all his promises and will never forget his people, so long as they will “confess their iniquity” and “accept of the punishment of their iniquity” so that “I might be their God,” and I will never forget them (Leviticus 26:40–42, 45) and that “the covenant of the Father may be fulfilled which he hath covenanted with his people, O house of Israel” (3 Nephi 21:1).
Book of Mormon Central, “What Makes 3 Nephi the Holy of Holies of the Book of Mormon? (3 Nephi 14:13–14),” KnoWhy 206 (October 11, 2016).