Moroni shifts his focus. He has been speaking of faith, but a faith that looked to Jesus Christ. Now, he speaks of those who have had faith, and have been able to see the future. One of the things that they have seen is the coming forth of the Book of Mormon. Moroni now focuses on that project on which he is working.
Looking forward to that time, Moroni is concerned that what he and his father have written might not effectively communicate the message. The message is that the Gentiles should take these words to those who would be called Lamanites. Moroni addresses his lament to God, and worries that “the Gentiles will mock at these things, because of our weakness in writing.” He declares that he and the other prophets have sufficient faith, but that their faith might not be communicated in the more impersonal medium of writing.
Moroni has read the vision of the future that the brother of Jared wrote, but which he sealed up. We do not have it to be able to verify that the brother of Jared was so much better at writing about faith than were Mormon or Moroni. We now have what Mormon and Moroni wrote, and even though they feared that their writings might not be sufficient, millions now have faith in them, and use them to spiritually guide their lives.