Mosiah 25:11 - 25:15
Mosiah 25:11 And again, when they thought upon the Lamanites, who were their brethren, of their sinful and polluted state, they were filled with pain and anguish for the welfare of their souls.
At the same time they were sorry for the Lamanites and their lack of spiritual direction. They were truly concerned for their condition.
Mosiah 25:12 12 And it came to pass that those who were the children of Amulon and his brethren, who had taken to wife the daughters of the Lamanites, were displeased with the conduct of their fathers, and they would no longer be called by the names of their fathers, therefore they took upon themselves the name of Nephi, that they might be called the children of Nephi and be numbered among those who were called Nephites.
Apparently some of Amulon’s followers had deserted him and went with Alma as he fled from Amulon. They were so dissatisfied with Amulon and the rest of the priest's conduct, particularly in the kidnapping of the Lamanite daughters, that some of their children that fled with Alma, refused to be called Amulonites. They wanted to be called Nephites and be part of the Nephite culture and not to any longer associated with Amulon.
Mosiah 25:13 And now all the people of Zarahemla were numbered with the Nephites, and this because the kingdom had been conferred upon none but those who were descendants of Nephi.
Now everyone under the rule of Mosiah were called Nephites. Because only Nephites could be kings. So everyone regardless of their ancestry were called Nephites because they were under the rule of the descendents of Nephi.
Mosiah 25:14 And now it came to pass that when Mosiah had made an end of speaking and reading to the people, he desired that Alma should also speak to the people.
After Mosiah’s speaking, he had Alma stand before the people and speak to them.
Mosiah 25:15 And Alma did speak unto them, when they were assembled together in large bodies, and he went from one body to another, preaching unto the people repentance and faith on the Lord.
Since they were not in one body, he went to each one to preach faith on the Lord and to repent of their sins.