Textual: It is difficult to discern what Mormon was reading that allows him to write this conclusion.
Certainly the Lord was capable of answering prayer, but the specificity of this answer suggests that it was an answer that perhaps Alma received, and all of his people understood as an answer to their prayer.
Had each individual received this precise answer, it would be nearly impossible to elicit such a response from the people in order to write it down. It is more likely that this is Alma's answer, and one that he understood from conversation with his people that their faith had already led them to.