Jesus establishes the authority of the twelve to baptize beyond those who were present. It was undeniable that those who were present would all desire baptism, for they were in Christ’s presence. However, there would come a time when Christ would not be with them, and yet people would need to repent and believe. Therefore, Christ notes that they, too, are blessed. They will believe upon the words of those who testify that they have seen him, even if they see him not.
The intent is an echo of what we see in John 20:29: “Jesus saith unto him, Thomas, because thou hast seen me, thou hast believed: blessed are they that have not seen, and yet have believed.”
After this verse, the text follows Matthew 5. Orson Pratt made his divisions of these chapters so that the Book of Mormon sections that closely follow the Sermon on the Mount would be easily compared. The 1830 text did not tie chapter endings to the endings of New Testament chapters.