Nephi has reached the limit of what he can say. He has alluded to the Second Comforter, or the appearance of Christ to you in the flesh, but then his message ends. He "cannot say more." But he has told you that when Christ appears to you that you should do what Christ tells you to do.
Then Nephi laments our unbelief, wickedness, ignorance and stiffneckedness. You have already been told that in the vocabulary of the Book of Mormon the word "unbelief" means that you do not understand correct doctrine. You accept false notions, or your understanding is so incomplete as to make it wrong.
What is "wicked" about not following the "doctrine of Christ" so that you can receive the tongue of an angel? What is wicked about not pressing forward in the light of the Holy Ghost to the point where you receive Christ in the flesh? Why would that failing be "wicked?" (D&C 88: 33.)
Why are we "ignorant?" Is it because of our lack of learning or sophistication, or instead because of it? Studied ignorance is the most indelible kind. It prevents someone from ever casting away unbelief. It enshrines unbelief.
These conditions are all culminated by "stiffneckedness." Meaning that we are not only in error, but we are decidedly committed to remaining so. We won't budge. Won't humble ourselves and ask the Lord to remove our scales of darkness. We just remain devoted disciples of unbelief, leading in turn to our wickedness, borne upon the shoulders of our ignorance. What a spectacle we are when seen in the light of the Holy Ghost - that is, through the eyes of a prophet like Nephi.
What is interesting is this comment comes at the very end of Nephi's ministry. It is an aged prophet carving his last message targeted to the last day audience of first gentiles, then secondarily the remnant, and finally the Jews. And to this latter-day audience beginning with us, Nephi is rebuking us. It must be because of his love for us. It must be motivated by the love of Christ, because it follows immediately after explaining to us the "doctrine of Christ." So whether it seems to be the case or not, this is a loving, kind, light-filled warning from someone who knows what we lack.
Soberly, however, this rebuke should be compared to the rebuke he leveled at Laman and Lemuel. He told them to stop debating the meaning of a revelation given to their father, and start asking God for answers. Compare Nephi's earlier warning and rebuke to his brothers with this verse addressed to us:
"And they said: Behold, we cannot understand the words which our father hath spoken concerning the natural branches of the olive-tree, and also concerning the Gentiles. And I said unto them: Have ye inquired of the Lord? And they said unto me: We have not; for the Lord maketh no such thing known unto us. Behold, I said unto them: How is it that ye do not keep the commandments of the Lord? How is it that ye will perish, because of the hardness of your hearts? Do ye not remember the things which the Lord hath said?—If ye will not harden your hearts, and ask me in faith, believing that ye shall receive, with diligence in keeping my commandments, surely these things shall be made known unto you." (1 Nephi 15: 7-11.)
We read that and think ourselves better than Laman and Lemuel because we identify ourselves with Nephi. Nephi, on the other hand, sees our day, and identifies us with Laman and Lemuel. What a profound disconnect our arrogance causes between Nephi's meaning and our reading.
He is being as plain as words can be. And we are being as obstinate and obtuse as unbelief, wickedness, ignorance and stiffneckedness can cause. You can feel the irony.