Moroni acknowledges the relationship between the miracles that we see in our lives and our faith that Yahweh will provide them. Although Yahweh does not change (and hence is always a God of miracles), human faith can vary greatly. When humankind departs from Yahweh’s way, that essential connection between faith and miracles is broken. The great miracle of the atonement remains, but the smaller miracles appear fewer. Moroni has personally witnessed the tremendous contrast between the God of miracles and faithless human beings. Moroni had experienced the ministration of the miraculously preserved Three Nephites (Morm. 8:11), who were now more than four hundred years old. An even greater miracle was that the Savior himself had appeared to Moroni (Morm. 8:35). Moroni knows that Yahweh continues to be a God of miracles because he has seen those miracles.
However, he has also seen his people fall away, and certainly miracles had ceased among them. Defeating the invading Lamanites had been temporal and temporary. The result of such general faithlessness was the absence of any miracle that deflected the Nephites’ final destruction. No wonder Moroni finds this argument poignant.