Verses 10-13 continue the brief development of the Nephite return to goodness and prosperity. The descriptions Mormon gives are generic, not specific events. His intent is not historical, but symbolic and structural. One of the first indications of this type of purpose is his use of “an exceedingly fair and delightsome people.” This statement is placed here to show the effect of the visit of the Lord. Mormon is repeated standard Nephite prophetic language. The idea of a fair and delightsome, or white and delightsome, people, has long been an image of righteousness. When Mormon uses that phrase in this context, he is showing both the righteousness of the people, and the fulfillment of prophecy:
2 Nephi 30:6-7
6 And then shall they rejoice; for they shall know that it is a blessing unto them from the hand of God; and their scales of darkness shall begin to fall from their eyes; and many generations shall not pass away among them, save they shall be a pure and a delightsome people.
7 And it shall come to pass that the Jews which are scattered also shall begin to believe in Christ; and they shall begin to gather in upon the face of the land; and as many as shall believe in Christ shall also become a delightsome people.
While we do not know if this is the particular prophecy to which Mormon alludes, it is quite appropriate. The reference to the shift from the Law of Moses to the Gospel of Christ simply highlights the continuation of the new covenant made upon the Messiah’s visit to the New World.