The mothers of these young men were possessed of a tremendous faith. They had tempered their faith in the face of certain death when their fellow Lamanites had come upon them in the land of Nephi. These mothers had faith in God sufficient to keep their oath to God even when it might have meant their life. That incredibly strong faith was passed on to their sons, but in a slightly different manner. Their mothers had taught them that God would deliver them if they were faithful.
This teaching is so similar to the Nephite foundational promise that we may expect that it is part of that promise. The Lord had promised the people of Lehi that if they were righteous that they would be preserved in the land. This promise was adopted by the Nephites as the one of the fundamental underpinnings of their collective social consciousness. These mothers among the people of Ammon clearly took this promise literally, and impressed it upon their sons. If God had said that his people would be preserved upon righteousness, then their sons would be preserved upon that same righteousness.
Certainly the sons had no doubt that their mothers knew faith, for they had certainly been told the stories of the faith of their parents in the land of Nephi, a faith that was reaffirmed in the difficult decision to maintain their oath even in the face of such great need. The tremendous faith of their parents was what led these sons into battle in the first place, and they were willing to continue to act upon that faith which they had learned at their mothers’ knees.