The final admonition to the community of believers is intended to be a summation of all that had gone before.
The beginning of the Sermon was the Beatitudes which indicate that the community were blessed from God, even if their worldly circumstances were not optimum.
The next section is the Antitheses, or the “they said…. I say” section. In this section Jesus lays out the way in which horizontal relationships differ form the Law of Moses to the Law of the Gospel.
The next section deals with the vertical relationships with God, and the nature of the changes required by the Law of the Gospel in the way in which we perform our devotions to God.
The final section deals with the pragmatics of living the gospel. Jesus tells the people the types of concerns they should, and should not have. He tells them about their ability to approach God their Father in prayer, and he gives them a foretaste of the way their community would fit into the greater world.
The concluding statement tells the community to build on the foundation of these teachings. The gospel has been laid out before them, and the way of the gospel explained. Now they must live it.
The imagery is of building. Jesus was a carpenter, and doubtless knew something of building. There were probably others in the audience who had similar skills, and there would have been even more that were at least familiar with the principle. Jesus notes the structural stability of a building built upon rock as opposed to one built on sand. The implication is that the principles of the gospel are the bedrock, and that they would provide the firm foundation for their faith and action.
Book of Mormon context: This imagery has already been used in the Book of Mormon, and certainly the Mesoamerican Nephites were familiar with both building and the need for solid foundations. Similar language is found in 2 Nephi 28:28 and Helaman 5:12.
Textual: There are no changes to the Matthean text in the verses that are included, but there are two verses that have been removed from the Matthean text.
Matthew 7:28-9
28 And it came to pass, when Jesus had ended these sayings, the people were astonished at his doctrine:
29 For he taught them as one having authority, and not as the scribes.
These verses conclude the Sermon in the Matthean text, and they also conclude the experience. In the 3 Nephi redaction there is an end of the chapter, indicating a termination of the sermon, but there is no termination of the experience. Jesus remains with the people after giving whatever version of the Sermon he gave to the Nephites at this time.