The phrase “they believed in Christ and worshiped the Father in his name” refers to the last part of verse 4, “all the holy prophets which were before us.” As Jacob discusses those prophets we have a curious repetition: “… worshiped the Father in his name, and also we worship the Father in his name.” The specific declaration of the worship of the Father in the name of the Messiah is problematic because it reflects such a modern understanding of the Father and the Son. Lehi believed (see 1 Nephi, Part 1: Context, Chapter 1, “The Historical Setting of 1 Nephi”) in an Atoning Messiah who would be the physical presence of Yahweh on earth. Nevertheless, Lehi also understood that Yahweh’s father, El, presided over the council of the gods in heaven.
Jacob may mean that he understands they worship El through the name of the Messiah (a title for the son of El, Yahweh). However, the Father/Son relationship is also applied directly to Yahweh the Messiah in the Book of Mormon. Standard Book of Mormon usage (at least prior to the Messiah’s appearance in Bountiful) understands that the Father is Yahweh while he is in his celestial realm, and the Messiah is Son because of the change of location to earth. Those terms indicate a change of location only, as Yahweh and Messiah are the same person. (See “Excursus: The Nephite Understanding of God,” following 1 Nephi 11 for a more complete discussion of Yahweh as both father and son.)
At this point, Jacob could have been articulating an awareness, apparently held by his father, Lehi, of both El and Yahweh. However, in the Israel of around 600 B.C., the understanding and worship of El was fading in preference to worshipping Yahweh (though retaining an understanding of El as Yahweh’s father). I suspect that the same dynamic was occurring among the Nephites and that Jacob would have referred to the Yahweh-Messiah, not the El/Messiah, relationship. I see it as at least possible that this particular phrase came more from Joseph Smith than from the plates.
Jacob next declares that keeping the law of Moses is a discipline to point them to Christ. This understanding resembles the Apostle Paul’s explanation: “Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith” (Gal. 3:24). Jacob equates living the law of Moses to Abraham’s test in sacrificing Isaac. Did Jacob think that the law of Moses was as hard to obey as Abraham felt about the commandment to sacrifice his son?
Jacob probably uses the Abraham story because it points to ultimate salvation. Abraham’s willingness to sacrifice Isaac resulted in Isaac’s salvation from literal death; the Messiah’s atoning sacrifice also leads to salvation from death (both physical and spiritual) for all of us.