Most of all, Jacob wanted his posterity to know that they knew of the coming of Christ. So Jacob emphasized in Jacob 4:4 that his people indeed "knew of Christ" and that they "had a hope of his glory many hundred years before his coming; and not only we ourselves had a hope of his glory, but also all the holy prophets which were before us." He also says that they searched the prophets and had many revelations that witnessed of the powers of the name and grace and condescension of Jesus.
All this sets the stage for his quoting of Zenos in chapter 5. That is a detailed prophecy, one which Jacob’s people could place confidence in because during their day it was already partially fulfilled. For example, it talks about the olive tree branches being scattered to the nethermost parts of the region and how one of those trees, planted on a "good spot of ground," was divided so that only some of its fruit was "tame" and the other fruit was "wild" (Jacob 5:25).
I think Jacob and his people could already see themselves as that tree that had been planted far away and how the Nephites and Lamanites had indeed separated. Well, if they knew it had been fulfilled that much, then they could trust that the rest of the prophecy was going to come true as well. They could also trust that the Lord of the vineyard will come and the harvesters and workers will come and all of the things that will occur as the Lord tries to restore Israel. They could trust that the Lord will do all he can to save the good fruit of the house of Israel and eventually restore the tree so that it can again bear the kind of fruit that he wants.
Book of Mormon Central, "What Do We Learn About Ministering from the Account of Sherem? (Jacob 7:15)," KnoWhy 534 (October 3, 2019).
Book of Mormon Central, "How Abraham’s Sacrifice of Isaac Illuminates the Atonement (Jacob 4:5)," KnoWhy 412 (March 1, 2018).