3 Nephi 4 - 7

As Mormon abridges the Nephite records of the tribulations, conflicts, teachings, prophecies, and so forth, amongst the people, he feels inspired at times to interject his some of his thoughts and feelings as well as some explanations for the benefit of the reader. After the signs of the birth of the Savior, Mormon writes -

"...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..." (Book of Mormon 3 Nephi 5:8)

But even though they are not included in this book he tells us that -

"...there are records which do contain all the proceedings of this people; and a shorter but true account was given by Nephi." (Book of Mormon 3 Nephi 5:9)

And then at this point in his writings he interjects -

"...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.
13 Behold, I am a disciple of Jesus Christ, the Son of God. I have been called of him to declare his word among his people, that they might have everlasting life..." (Book of Mormon 3 Nephi 5:12 - 13)

And because he was commanded by the Lord -

"...I do make the record on plates which I have made with mine own hands..." (Book of Mormon 3 Nephi 5:11)

He explains here that it -

"...hath become expedient that I, according to the will of God, that the prayers of those who have gone hence, who were the holy ones, should be fulfilled according to their faith, should make a record of these things which have been done—
15 Yea, a small record of that which hath taken place from the time that Lehi left Jerusalem, even down until the present time.
16 Therefore I do make my record from the accounts which have been given by those who were before me, until the commencement of my day." (Book of Mormon 3 Nephi 5:14 - 16)

"And then I do make a record of the things which I have seen with mine own eyes." (Book of Mormon 3 Nephi 5:17)

Even though this work is commanded by the Lord, I'm sure it wasn't necessarily easy to accomplish as he tells us -

"...I know the record which I make to be a just and a true record; nevertheless there are many things which, according to our language, we are not able to write." (Book of Mormon 3 Nephi 5:18)

Mormon seems to understand that his writings are intended for future generations as he testifies -

"I am Mormon, and a pure descendant of Lehi. I have reason to bless my God and my Savior Jesus Christ, that he brought our fathers out of the land of Jerusalem, (and no one knew it save it were himself and those whom he brought out of that land) and that he hath given me and my people so much knowledge unto the salvation of our souls." (Book of Mormon 3 Nephi 5:20)

"Surely he hath blessed the house of Jacob, and hath been merciful unto the seed of Joseph...
23 Yea, and surely shall he again bring a remnant of the seed of Joseph to the knowledge of the Lord their God.
24 And as surely as the Lord liveth, will he gather in from the four quarters of the earth all the remnant of the seed of Jacob, who are scattered abroad upon all the face of the earth.
25 And as he hath covenanted with all the house of Jacob, even so shall the covenant wherewith he hath covenanted with the house of Jacob be fulfilled in his own due time, unto the restoring all the house of Jacob unto the knowledge of the covenant that he hath covenanted with them.
26 And then shall they know their Redeemer, who is Jesus Christ, the Son of God; and then shall they be gathered in from the four quarters of the earth unto their own lands, from whence they have been dispersed; yea, as the Lord liveth so shall it be. Amen." (Book of Mormon 3 Nephi 5:21 - 26)