Moroni uses as his example the most important witness that his world had known, the appearance of the Atoning Messiah at Bountiful. There were many in the world prior to the appearance of the Savior who did not believe, and Moroni notes that. However, there were yet those who remained faithful. At this point, Moroni assumes that his readers already know this story. He does not repeat it fully, but merely as passing reference.
He knows the story very well, and simply assumes that we will know it as well. In particular, the current context requires that we remember that the faithful were sorely persecuted prior to the appearance of the Savior. This is Moroni’s point. They had faith, and their faith went through severe trials. Nevertheless, after the trial of their persecutions, came the glorious appearance of the Savior that justified all of their faith.
It is interesting that while he springboards to this discussion on faith from the experience of the Jaredites, it is upon the experience of the Nephites that he builds his case.