If the purpose of the Book of Mormon is to persuade the world that Jesus is truly the Christ, one might ask, Why did Mormon not continue to write in detail of Christ's Nephite ministry? According to Donl Peterson, the answer is in the text: Mormon wanted to continue; he had planned to, but the Lord forbade him. Mormon explained the situation as follows:
And these things have I written, which are a lesser part of the things which he taught the people; and I have written them to the intent that they may be brought again unto this people, from the Gentiles, according to the words which Jesus hath spoken. And when they shall have received this, which is expedient that they should have first, to try their faith, and if it shall so be that they shall believe these things then shall the greater things be made manifest unto them. And if it so be that they will not believe these things, then shall the greater things be withheld from them, unto their condemnation. Behold, I was about to write them, all which were engraven upon the plates of Nephi, but the Lord forbade it, saying: I will try the faith of my people. (3 Nephi 26:8-11; emphasis added). [H. Donl Peterson, Moroni: Ancient Prophet Modern Messenger, pp. 12-13]
Note* Students of the Book of Mormon would do well to remember the Lord's counsel to Mormon here. When various pieces of textual data concerning culture, chronology, history, etc. do not seem to fall into place immediately, or if the lack thereof seems to frustrate a more complete understanding of the text, it might be because the Lord inspired Mormon to purposely leave out those details. In essence, the Lord might be trying our faith. [Alan C. Miner, Personal Notes]