Jacob’s introduction is interesting. He says that he will speak of things that are, and are to come—and therefore will read the words of Isaiah. Isaiah was a prophet from one hundred years in the past, and in a different world. Nevertheless, Nephi sees them as relevant because Nephi gave Jacob the assignment to speak these words. Although Isaiah was well in the past, Nephi wants Jacob to read Isaiah because there is an application to those who will hear the sermon. It is “for your sakes.” That is what Jacob means when he says of the things that are. They are things relevant to their current life. That there are aspects which speak of the future is also true, but Nephi doesn’t ask Jacob to speak of an esoteric occurrence over a thousand years in their future. It is relevant now.
That current relevance is reiterated when Jacob reminds them that Isaiah spoke to all of the house of Israel. Since they are also of the house of Israel, Isaiah is applicable to them. It will be important to the understanding of these chapters of Isaiah that we remember that the Nephites now consist of the descendants of Lehi, mixed with an undoubtedly larger population of people who are of the house of Israel by adoption, rather than birth. Nevertheless, that adoption placed them as equivalent to the natural born descendants. Isaiah is relevant to them, even though they might be adopted into Israel.