For their salvation is necessary and essential to our salvation, as Paul says concerning the fathers—that they without us cannot be made perfect—neither can we without our dead be made perfect. Doctrine & Covenants 128:15
Who did Ammon say had a faith that was greater than that of any Nephite?
King Lamoni's wife
Ammon said unto her [the wife of King Lamoni]: Blessed art thou because of thy exceeding faith; I say unto thee, woman,
there has not been such great faith among all the people of the Nephites. Alma 19:10
Because she believed him when he said that her husband (who appeared to be dead for several days and was beginning to stink)
was still alive.