Robert Millet notes that Jacob 5, the allegory of Zenos, is the longest chapter in the Book of Mormon. And who is being quoted to give such a message? A prophet of the brass plates, Zenos. What is Jacob's purpose in quoting Zenos? To give a panoramic vision of the destiny of the house of Israel, from premortal times to millennial times. It is the things God has in store for his chosen people. As Jacob declares in chapter 4, "I will unfold this mystery unto you" (Jacob 4:18). Millet then asks, What is the message of the allegory of Zenos? That God just will not let covenant Israel go. God will honor his covenants. He will work with her, and he will cut that tree, and he will prune that tree, and he will dung that tree, and he will do whatever it takes. Look in chapter 6 and see if this isn't a great summary. It isn't that we have to know the particulars of Jacob 5. (I hope we don't to be saved! I know a couple of them.) But it's maybe the greater message of Jacob 6:4-7 [that we need to know]:
And how merciful is our God unto us, for he remembereth the house of Israel, both roots and branches; and he stretches forth his hands unto them all the day long; and they are a stiffnecked and a gainsaying people; but as many as will not harden their hearts shall be saved in the kingdom of God.
Wherefore, my beloved brethren, I beseech of you in words of soberness that ye would repent, and come with full purpose of heart, and cleave unto God as he cleaveth unto you. And while his arm of mercy is extended towards you in the light of the day, harden not your hearts.
Yea, today, if ye will hear his voice, harden not your hearts; for why will ye die?
For behold, after ye have been nourished by the good word of God all the day long, will ye bring forth evil fruit, that ye must be hewn down and cast into the fire?
There really is the central message of the allegory.
Remember, that one of the major purposes of the Book of Mormon was to establish the essential truthfulness of the Bible (see 1 Nephi 13:40). For many years, you know, we have approached this in the wrong direction. We have gone out trying to prove the Book of Mormon from the Bible. The Lord never intended that. The Lord's purpose is that he knew there would come a time when the Bible itself would be in question. So he raises up a prophet, restores to him ancient records, gives him power to translate them, and they come forth and help establish essential truthfulness. That is why the Lord would say, in the 20th section of the Doctrine and Covenants, that the Book of Mormon is given for the purpose of proving to the world that the holy scriptures are true. [Robert L. Millet, "The Prophets of the Brass Plates," Video Transcript, FARMS, 1996, pp. 10-12, 3-4] [See the commentary on 1 Nephi 13:40; Alma 33:11; Helaman 8:17]