In the last episode, verse 9 listed the reason the Nephites had prospered. They understood the promise of the land. Verse 10 follows that declared reason by noting that this is what the prophets preached. It is interesting that for Lehi and Nephi the promise was stated as a positive. The Nephites would prosper if they obeyed the commandments. Here, the prophets warn the people that if they do not keep the commandments, they would be destroyed. These are the two sides of the same covenant and promise of the land, but apparently the negative statement was more relevant to the Nephites of the time.
What did it mean to live righteously? From the plates of brass they understood that one part of the answer was to live according to the law of Moses. However, also according to the brass plates, but reinforced by Nephi, Jacob, and other unnamed prophets, was the idea that they should look to the coming atoning Messiah. Following those two teachings would prevent them from being destroyed.