3Ne Ch26

Jesus explains the words He had the multitude write from beginning to end, but not all is written in records for us because we must ask for those after a trial of our faith. The Lord performs miracles, teaches, and gives the sacrament many times while He is among the Nephites.

The reason He told the people these words was because it was not apart of the scripture they had and He wanted them to know to give unto future generations including the explanation of all things from the beginning of time until the Christ's glorious coming (v1-3). The great and last day will come when all nations and tongues shall stand before God to be judged of their works being good and getting everlasting life, or being evil and doomed to everlasting damnation (v4-5). There are many more things that Christ did teach the people but they cannot all be written on the plates of Nephi, but they contain the most that was written and Mormon wrote as much as he did in the Book of Mormon so we can read and have a test of our faith to see if we can receive greater things from God when we are ready (v6-9).
From this we know that the Lord wants to test our faith and He will reward though who are faithful and those who do not understand the words in the Book of Mormon will not have a chance to learn more (v10-12). The Lord is among the Nephites for 3 days administering the sacrament often, teaching, and performing miracles such as loosening the tongues of the children who speak marvelous things before He again ascended into heaven (v13-15). The children's tongues stayed loosed and they did speak and teach marvelous things as people gathered to hear them, but we do not know what they spoke because they are forbidden to be written (v16).
The 12 disciples continued to teach Christ's words around to everyone and they saw many miracles that are not supposed to be written (v17-18). The people lived in peace, dealing justly and sharing everything knowing that this was the way that Jesus had told them to live with one another (v19-20). The church was established and everyone was baptized to be a part of the Church of Christ, for that is what it is called and we will learn why in the next chapter (v21).