In this verse, Mormon pulls together the threads of his discourse. Since miracles come by faith, and there are miracles, then therefore there is also faith. This is God’s plan: that we have faith and that our faith have reasonable objects upon which to rest.
If there are those who think that miracles “have ceased” (or for whom they really have ceased), then this condition is evidence of “unbelief.” Such a situation is indeed a sorry state for humankind (“all is vain”). Without faith and without miracles, then we are lost, for there is no way to lay hold of the “good things” without faith.