The Nephite community is being divided between the righteous and the unrighteous. Jarom understands the promise of the land, and notes that it is only God’s exceeding mercy that has prevented them from being swept from the face of the land. Of course, there were also some who were righteous, and they also entered into God’s mercy in not yet allowing the destruction of the Nephites.
How did this happen so quickly? How does it begin in the lifetime of Nephi and Jacob, who were born in the Old World? In the ancient world there were no such things as religions that one would belong to, such as define modern concepts. They were simply the definitions of the way things were. If the Nephites had firmly established the understanding of what was really behind the way of the world, we would not expect it to so quickly disappear. That is, unless they were competing with other ideas about how things ought to be seen.
Understanding the Nephites as a population that included unnamed, but inferred, others—provides the answers. Just as Nephi and Jacob brought with them the ideas of the Old World, these others would have brought their ideas about how things were from their own New World background. As they continued to live in the New World, and were surrounded by peoples holding to competing ideas, it is easy to see how those non-Israelite ideas might influence and tempt away the Nephites who now lived in that environment, and many of whom had been converted from it.