In his capacity as disciple, or apostle, Mormon declares that he has made “a just and true record.” It is interesting that Mormon understands that it can be “just and true” even though it is not comprehensive. He has selected those things that fit the message he wanted to deliver.
In verse 19 he says that “now I make an end of my saying,” but he is only beginning to end what he has to say. Mormon is writing off-script at this point. Verse 19 tells us that he knows that he needs to return to the planned text, but he still has more on his mind before he does.
The beginning of this final statement (at this time) is a repetition of who he is. He declares that he is a “pure descendant of Lehi.” There is no way to fully know what he means. He might trace his genealogy through one of the tribes of Lehi’s sons, but it is unclear how any lineage would be considered “pure” after a thousand years. The concept is probably not genetic, or even political. Mormon has said that he was to write about the Nephites from the time Lehi left Jerusalem, and Mormon the apostle is connecting himself as an ideologically, or religiously pure, descendant of Lehi, and therefore drawing a straight line from Lehi’s righteousness to Mormon as an apostle.