There are two clauses in this verse, and both require careful examination. in the first clause we are to understand that Nephi's people do not understand Isaiah because Nephi has deliberately withheld potential information. How might he do this? If the Nephites consist of a large majority of people who crossed from the Old World to t he New, then their Jewish acculturation would already have taken place. If we include those children born in the wilderness, then the lack of learning would be placed at Lehi's feet not Nephi's.
Nephi is very clearly speaking of his people, and his charge in their education. That Nephi is not speaking to a future uneducated population is evident by his admission that the fault lies in his own choice of what to teach and not teach. Nephi is not saying that he knows prophetically that future generations will not understand Isaiah, but rather that his current generation does not understand, and that it is specifically due to a choice he has made.
The disconnection between generations does not appear to be sufficient to warrant Nephi's recognition that the manner of prophesying of the Jews, indeed many aspects of Judaism, have not been continued. The Old World parents of Nephi's generation would have even unconsciously transmitted much of that culture without regard to Nephi's teaching. Nephi assumes a key role in the transmission of cultural information precisely in the case posited for Jacob's speech in 2 Nephi 6, a situation where a large number of gentiles are being admitted into Nephite society. Under that scenario, Nephi becomes the controller of the types of cultural information passed on (by virtue of his position as King) as well as the formulator of a new social order based on his understanding of the gospel rather than inherited Jewish traditions.
The second phrase of the verse is more problematic for its vehemence. Nephi states: "...for their works were works of darkness, and their doings were doings of abominations." Nephi gives us no clue as to his meaning here. Surely he did not condemn his entire heritage though he certainly has jettisoned some part of it. What might he mean?
In the context of the phrase's position in a transitional passage between a long citation of Isaiah and the coming improvisation thereon, it is probable that Nephi's condemnation of his cultural roots is related to Isaiah's condemnations which have been occupying Nephi's pen for however long it takes to copy passages onto plates - a process that must be cumbersome in any case, but increased in intensity by the necessity of accurately entering copying text rather than inventing it. Of course it cannot be certain that Nephi is speaking only on the basis of Isaiah, as his father also suffered rejection at the hands of the Jews. Surely this had something to do with his feelings.
Perhaps one of the clues to Nephi's contextual reference to Isaiah comes from the construction of the condemnation itself. As is frequent in Isaiah, Nephi's phrasing is a nice dual parallel, with two phrases similarly constructed with identical meanings used to emphasize the thought. While Nephi can produce beautiful literature, witnessed by the Song of Nephi (2 Nephi 4:16-35) he does so infrequently, preferring a rather plain prose. This paired parallel construction coming so soon after the long and laborious process of copying Isaiah is highly suggestive that Nephi's comments should be seen in the light of Isaiah's criticisms.
What then are the works of darkness? As with Isaiah, Nephi does not criticize the Lord, nor the Law. With the emphasis on "works" and "doings" Nephi is condemning the ways in which the Jews performed those actions of the Law. Like Isaiah, he would condemn those who did not understand the justice of God, and who would deny rights and justice to the poor, a theme that is later highlighted in King Benjamin's great speech.
Nevertheless, Nephi specifically notes that one of the things that has been lost is the "manner of prophesying." Why, along with the works of darkness he condemns, would the "manner of prophesying" also be lost?
The answer is necessarily speculative, as Nephi does not give us enough information to be sure. Nephi had, in his youth, had a tremendous experience with the spirit when he wanted to know more of what his father had seen in a vision. While that experience was transforming, it is also clear that Nephi didn't understand his father until he had seen his own vision. It is equally certain that his brothers did not understand it either.
Lehi was a prophet of the Old World, a "visionary man," one given to precisely the "manner of prophesying" of the Jews, and of Isaiah. Nephi's prophetic style never became "visionary," even after his own vision. While not specifically connected to the "works of darkness," this visionary, symbolic, approach to prophesying is certainly one that Nephi discarded to provide his own preference for greater plainness and clarity, a preference that colors the remainder of Nephite prophetic history.
Narrative: Nephi begins with a logical transition. Because he has presented so much text he needs to provide some linkage between that text and his elaboration of it. Even though it provides a needed transition, the development of the transitional text begins to take on a life of its own. and the transition becomes not a linkage between source scripture and elaboration, but a mini-discourse on Nephi's perceptions of both his historical roots in Jerusalem and is lifetime efforts to lead a people in the New World. As noted in the introduction to this section, we see more rambling in Nephi's narrative, and it begins with this transition that takes a flight of its own rather than move quickly to the point of scriptural elaboration.