Aminadab is the man who had been born a Nephite, and who clearly remembered what he had been taught of the Nephite religion. Seeing the destruction around them, and the miraculous preservation of Nephi and Lehi, the Lamanites ask how they might survive this calamity. When they ask “what shall we do, that this cloud of darkness may be removed from overshadowing us,” Mormon provides a phrase with two meanings. One would be salvation from the physical conditions that led to the destruction of the walls and the physical cloud of darkness. However, there is the second meaning: they have been under a cloud of darkness because they knew not God.
Thus, Aminadab tells them that the solution is to have faith in Christ. There is no teaching at this time, and Aminadab simply assumes that they know of Christ, and that there were some there who had heard Alma, Amulek, and Zeezrom preach of Him. This suggests that many of those present were apostate Nephites, even though they were in Lamanite lands.