With the missionary emphasis on Moroni 10:4–5, it might be excused if we think that Moroni was writing so that people would believe in the Book of Mormon. There is certainly an element of that, but Moroni understood the Book of Mormon as a book. We are not to believe in the book, but rather in the theme of the book, which is to testify of Christ. Why did Moroni depart from the exhortation of seeing if “these things are not true” and then turn to discussion of the gifts of the Spirit? In fact, he didn’t change course at all.
When Moroni exhorts us to ask God if “these things are not true,” Moroni was speaking of the things of Christ. He did not intend that verse to be as exclusively applied as our missionary efforts make it. Therefore, right after he exhorts us to ask, he declares that “whatsoever thing is good is just and true, wherefore, nothing that is good denieth the Christ, but acknowledgeth that he is” (see verse 6 of this chapter).
Right after speaking of the good that comes from Christ, Moroni speaks of some of the good things that come from Christ, the gifts of the Spirit. Thus, verse 18 confirms that “I would exhort you … that ye remember that every good gift cometh of Christ.”
Moroni’s message to future generations is contained in the Book of Mormon, but the message reflects the purpose of the book, not simply the existence of the book.