The idea of Mormon as narrator is a subtle complexity in the Book of Mormon (especially since he isn't the only narrator). Although 3 Nephi 5 isn't the only time Mormon breaks from the narrative to point things out to us ("and thus we see ... "), whenever he does, there is usually a good reason, which he tells us.
In this example, which comprises 3 Nephi 5:8-26, Mormon begins the aside with an explanation that his record is only a cursory summary of the history of the people, but he goes on to offer us several intriguing details about his life and the mission to which he has been called.
What follows in the rest of this post and all of part 2 is the entire passage (verses 8-26) interrupted by comments from me about what I consider to be the significant details we get about Mormon from these verses:
8 And there had many things transpired which, in the eyes of some, would be great and marvelous; nevertheless, they cannot all be written in this book; yea, this book cannot contain even a hundredth part of what was done among so many people in the space of twenty and five years;
9 But behold there are records which do contain all the proceedings of this people; and a shorter but true account was given by Nephi.
10 Therefore I have made my record of these things according to the record of Nephi, which was engraven on the plates which were called the plates of Nephi.
In verse 9, the Nephi who is mentioned is Nephi, who was the son of Nephi, who was the son of Helaman, who was the son of Helaman, who was the son of Alma, who was the son of Alma. (That's fun, isn't it?) Apparently this Nephi kept a more detailed record of the proceedings of the people along with the "record of Nephi" which was created by the original Nephi (see 1 Nephi 9:2 and 2 Nephi 4:14). Here we are 625 years after Lehi left Jerusalem and the record of Nephi is still being actively kept.
11 And behold, I do make the record on plates which I have made with mine own hands.
I consider this to be very significant. The original Nephi made plates (see 1 Nephi 1:17 and 1 Nephi 19:1). Other authors speak of running out of space on the records, implying they didn't have the means or knowledge to make more plates. See Jarom 1:14, for example.
12 And behold, I am called Mormon, being called after the land of Mormon, the land in which Alma did establish the church among the people, yea, the first church which was established among them after their transgression.
Verse 12 contains a great example of internal consistency. We know from Mosiah 18:4 that "the king" (possibly Noah?) had named a certain area "Mormon," and that Alma and his first followers fled there as a place of resort. We also know that it was in the waters of Mormon that these first believers were baptized. This was the beginning of a new church in the land. We also know that the name was significant to Mormon as he recorded this mass conversion experience (see Mosiah 18:30).
In part 2, we'll continue looking at this long and fascinating aside.
In part 2, we'll continue looking at this long and fascinating aside.