I always stress parsing what actually is said in written text or by someone we hear to understand what we can assert as definitive - and then considering the entire context to see if there are possible implications from whatever simply is a given (what is indisputable). For example, in Alma 56:47-48, all we are told is that the mothers “taught” their sons that “if they did not doubt, God would deliver them.” From what, we are not told. We only assume it was from physical death in war because of the situation (war and their preservation in it) that caused them to relate it to Helaman. However, the first two-thirds of verse 47 make it clear that they were willing to die. Given that, it is legitimate to look at the rest of the context and realize that there might be other legitimate meanings for “God would deliver them.”
First, "they had been taught” - which might have been a one time occurrence as they were leaving home. However, it seems like these young men "had been taught” dedication and obedience and exactness all their lives. Individuals can change in an instant, but it is unlikely that 2,000 young men suddenly would become super-righteous overnight. It is much more likely that they "had been taught” that God would deliver them from anything that might threaten their spiritual, eternal well-being than that their mothers simply pulled them aside on the way out the door and promised them they wouldn’t be killed in the war.
Remember, those mothers had seen many of their friends (and perhaps some fathers and sons and husbands) slaughtered by other Lamanites - killed in the act of calling upon God even though they did not “doubt God”. They knew full well that God didn’t always deliver His faithful people from physical death, but they were convinced that He could deliver them from their natural, fallen, sinful and lost state - from spiritual death. (Alma 24:27 - “thus we see that the Lord worketh in many ways to the salvation of his people.”)
Alma 53: 20-21 makes it obvious that these young men had been taught all their lives the reward for perfect faithfulness, obedience and dedication - the same reward their own “pioneers” had received, even those who had been killed for their faith, valor and lack of doubt. I think it’s fairly safe to guess, as a parent myself, that their mothers reiterated what they had been taught all their lives as they were heading off to war (that, if they did not doubt and obeyed every command with exactness, God would deliver them - no matter the physical outcome.)
Remember, this was Helaman who was reporting about these young men. He was as close to them as any Nephite - ever. Yet, apparently, he did not know the specifics about what their mothers had taught them until they told him about it. He had been there when the parents had decided to fight; he was the one who had talked them out of it by invoking their sacred promise; he had been chosen as their sons’ military leader, because they trusted him as a religious leader. He was intimately involved in the decision of their sons to fight in place of their parents.
Perhaps “taught” simply can mean “told” - but I tend to believe that Helaman would have been there for the great send off when all the mothers collectively told all the sons that they would not die in battle - that he would have known about it and not have to be told after the fact.
Having said all that, I do not discount the idea that the Lord promised the parents that He would preserve their sons in battle like He had promised Mosiah and Alma to preserve their sons on their missions to convert these very parents (a promise of which Helaman would have heard from his own father and understood intimately) - that is was couched in terms of “You’ve sacrificed enough lives to follow me. I won’t require that you sacrifice your children.” Even if that really is all it was, that’s enough for me, since it makes it an incredibly powerful story of the rewards of deep and difficult sacrifice and dedication.I am not saying that this is the correct view of the statement, "God would deliver them." It simply might have been, “Stay valiant and none of you will be killed.” All I’m saying is that when you parse the text, there is more than one possible meaning for that phrase (”God would deliver them.”) - and, to me, the more comprehensive one I describe here is much more powerful and moving and inspiring in its applicability to me and my own battles.