Verse 14 emphasizes that "the Jews were a stiffnecked people; and they despised the words of plainness, and killed the prophets, and sought for things that they could not understand." Perhaps Jacob had children or grandchildren who were wondering why their family had left Jerusalem. Well, the wickedness of the Jews provides an understandable answer.
And how does this relate to Jacob’s rehearsal of the Allegory of the Olive Tree? Well, what happens to the branches of the main tree, which is the tree in Jerusalem? It begins, right off the bat, to grow rotten. And it seems that Jacob wants his people to know that this was prophesied of long before any of them came on the scene.
But the Lord knew that the house of Israel would become sinful, and so he provided a way for branches to be cut off, transplanted, grafted in, and eventually brought together again. He would even remember those branches—like Lehi’s family—that were grafted into trees in the most remote parts of the vineyard (i.e. the world). I think it would have been comforting to Jacob and his people to know that, despite their physical separation from the land of Israel, they were still important to the master of the vineyard. They weren’t forgotten.
And, most of all, these people would eventually play an important role in the restoration of the house of Israel. Indeed, somehow those Jews who had "looked beyond the mark" would again be able to build upon "the only sure foundation" (4:16). How that would happen was a great "mystery" which Jacob now proposes to unfold to his people.
I think Jacob now has their undivided attention, and he has prepared them to hear the Allegory of the Olive Tree. Nevertheless, he is fearful for his audience. He may well be extremely anxious because, if they aren’t careful, they might find themselves among the branches that will be cast away for not bearing good fruit. And with that, Jacob turns to Zenos.