Nephi has seen in his vision that when the gospel is restored in the latter days, his own descendants and those who become converts will have certain questions and they will ponder them in their hearts. Therefore he says:
Nephi wants the converts to the Church in modern times to understand that once they get the Holy Ghost it will teach them the things that they should do in order to endure to the end and receive the fruits of their salvation. But not everyone knows how to use the Holy Ghost and so he says:
“Wherefore, now after I have spoken these words, if ye cannot understand them it will be because ye ask not, neither do ye knock; wherefore, ye are not brought into the light, but must perish in the dark. For behold, again I say unto you that if ye will enter in by the way, and receive the Holy Ghost, it will show unto you all things what ye should do. Behold, this is the doctrine of Christ, and there will be no more doctrine given until after he shall manifest himself unto you in the flesh. And when he shall manifest himself unto you in the flesh, the things which he shall say unto you shall ye observe to do.”