Here Nephi shows he is the prophetic precedent upon which others would build. We often quote Alma for this idea, because Alma lays it out more completely. But Alma's teaching has its root in Nephi's warning here. (See Alma 12: 9-11.)
Notice the Lord's promise that He will "give unto the children of men." He is liberal and does not upbraid us for asking. (James 1: 5-6.) Indeed, He admonishes us to seek, ask and knock.(Matt. 7: 7-11; D&C 88: 63.)
Notice that "unto him that receiveth" is the singular. It isn't "those" but "him." Meaning each of us individually may come to Him and ask and receive. However, "from them that shall say, We have enough" is plural. Meaning, there are many who could have been taught, had they not shut the doors. The collective will resist new revelation, even when it continues. They will shut their minds and not tolerate learning of truths.
When, however, groups close their minds it becomes impossible to keep what they have. It is inevitable that "from them shall be taken away even that which they have." Meaning, that we are either in a process of restoration or apostasy. The instant we stop restoring truths, we begin to lose them. You cannot just keep what Joseph restored to you. That will be lost. You either continue on and receive on-going revelation and new visions, visitations and restoration, or else you begin to forfeit what you already have. So soon as you walk away from one precept, from one doctrine, from one ordinance, you have begun the process of apostasy or falling away.
This environment is not static. It is always in change. Either it is being built up, created and newly formed, or it is decaying, dying and falling apart. It never holds static. This is because the things of this world testify of Christ's Gospel. (Moses 6: 63.) The path is only upward. So soon as you stray from it, you lose the path. This is why you take the shoes from off your feet and put them on again as a symbol that you accept the path. You never remove them again, because once upon that path you are to remain so. The path is accompanied by greater light and truth, going from grace to grace until you receive a fullness. (D&C 93: 19, 26-28.)
Nephi is telling us how to receive the fullness. It comes only through revelation and the opening of heaven. You may be anointed to become kings and priests, but the realization of these blessings depends upon your faithfulness. If you are true and faithful the time will come when you will be called up and anointed, whereas now it is all conditional. Only Christ can remove the conditions. To have Him do that requires Him, as the Word, to intercede on your behalf. It requires Him to confirm by His own voice from heaven that you are His, and to establish His covenant directly with you.
Whether it is the words of an old book or the words of an older ceremony it is the same. They can't save you. Only following the exact same path as the ancients followed can result in arriving at the same end. As the Sixth Lecture from the Lectures on Faith states:
What role does revelation play? Without it no person can be saved.
Who must receive the revelation? Each person for themselves.
What happens when revelation stops? To the person for whom revelation has ended, there is no salvation, (Moroni 7: 37-38) and they will immediately begin to lose what they were previously given. (Verse above.) To the people who refuse to receive more, "from them shall be taken away even that which they have."
Nephi has declared it, using authority from God to do so. We either accept his counsel and warnings or reject it at our peril.