God loves His prophets. He rejoices when someone will listen to Him and repeat what He says, and attribute the words to Him. These are wonderful things. Nephi is not a freak for teaching and testifying to these things, but he was a prophet loved by God. You should listen to him for he declares the words of God to us. He teaches truth. And those who teach His words are loved by God.
Not only does God love His spokesmen, but He will fulfill the things they teach in His name. Bearing testimony from God is a weighty matter, when the spokesman speaks the truth. Nephi has been doing so. Rejecting his teachings is perilous. His brothers reject him casually. By using Isaiah’s words, Nephi is able to rebuke and caution these rebellious brothers of the peril they face. He is also cautioning us. These words give comfort to Nephi, and to all those who know the truth and bear testimony of it. Truthful testimony, borne through the Spirit, will be words used to judge the hearers, and are always rejected at some risk. Joseph Smith commented
“neither Jew nor heathen can be culpable for rejecting the conflicting opinions of sectarianism, nor for rejecting any testimony but that which is sent of God, for as the preacher cannot preach except he be sent, so the hearer cannot believe without he hear a ‘sent’ preacher, and cannot be condemned for what he has not heard, and being without law, will have to be judged without law.” (Discourses of the Prophet Joseph Smith, compiled by Alma P. Burton, p.142.)
For those who receive Nephi’s words, however, they will be judged by the words Nephi has written. Any truthful testimony from any truthful witness is rejected at your peril. Here, however, the criteria are clear: we are accountable for Nephi’s testimony. We will be judged against that standard.