Nephi calls the people to repentance. The promise of the land is that they would prosper, and be protected, if they kept God’s commandment. They are not doing that, and, therefore, he tells them: “Except ye will repent, behold, he shall scatter you forth that ye shall become meat for dogs and wild beasts.” Of course, one of the problems with this call to repentance is that part of the promise was prosperity, and they appear to have prosperity under the Gadiantons; prosperity, but not equality.
When Alma spoke to his sons, and then when Helaman spoke to his sons (Nephi and Lehi), the two prophets emphasized remembering the captivity of the fathers. Nephi remembers it and attempts to remind those gathered that it has been their God who had delivered them in the past (see verse 20).