Mormon does not tell us the source of the Nephite belief, but this Doctrine and Covenants passage tells us that God had revealed it to Nephi (v. 32) and, even earlier, to the patriarchs: Joseph, Jacob, Isaac, and Abraham. Apparently it was a belief of the Nephite nation since its founding. The Doctrine and Covenants echoes the more concisely phrased passage in Alma 48, even to the promise that the Lord would “fight their battles” (v. 37) with them. This promise clearly parallels Alma 48:16.
Redaction: An interesting question is why this information appears at this point in the narrative. Mormon had just been describing Moroni and returns to Moroni in the next verse. I therefore read this passage as Moroni’s policy when fighting in defense of his people’s liberty. Mormon is presenting this information, not as a discussion of Nephite belief, but of Moroni’s faithfulness to those beliefs.