The phrase “the isles of the sea,” allows Jacob to turn from the Jews of the Old World to his people in the New World. He joins Nephi in applying this phrase as referring to the Nephites. (See commentary accompanying 1 Nephi 19:10.) Nephi inserts “isles of the sea” in his redaction of Isaiah 49: “Thus saith the Lord: In an acceptable time have I heard thee, O isles of the sea, and in a day of salvation have I helped thee; and I will preserve thee, and give thee my servant for a covenant of the people, to establish the earth, to cause to inherit the desolate heritages” (1 Ne. 21:8). This passage in KJV Isaiah 49:8 reads: “Thus saith the Lord, In an acceptable time have I heard thee, and in a day of salvation have I helped thee: and I will preserve thee, and give thee for a covenant of the people, to establish the earth, to cause to inherit the desolate heritages.”
Whether “isles of the sea” was on the brass plates or whether it was Nephi’s insertion/interpretation, both Nephi and Jacob interpreted the phrase as a prophecy about their people. Thus, Jacob quotes that phrase to introduce his discussion of the Nephites’ future.