These five verses are written directly to the future Gentile readership of the Book of Mormon. The promise is that if they believe in the Book of Mormon, that God “will show unto [them] the greater things, the knowledge which is hid up because of unbelief.” That statement appears to refer to the sealing of the record of the brother of Jared’s vision of the future of the world.
That assumption appears to be confirmed when verse 16 links those things to be revealed with those things “written by [Christ’s] servant John.” The coming forth of the Book of Mormon, therefore, becomes a sign that the events of the ending of days are being put into motion (verse 17).