The mystery that Alma reveals seems perplexingly simple. Why did it require such an elaborate introduction? The answer lies in that disjunction between Corianton’s present and the misty future in which the Messiah would come. We are living after Christ’s mission. The Messiah’s atonement is, from our perspective, a fact in our world. Alma and Corianton lived in a very different world and obviously struggled to understand how an atonement might be effective before it had taken place. How could those who lived (and died) before the atonement receive its benefit?
Alma testifies that all shall come forth from the dead. This is not an abstract theological issue, but literally a life-and-death issue for Alma and Corianton. They will go into the grave and see corruption; the resurrection—this promise of incorruption—will not happen for them until after the Messiah’s coming. They, as virtually all ancients, understood the long-term effect of dying and the decomposition of the body. Alma testifies that regardless of whether they lived and died before or after the Messiah’s mission, all will be resurrected. Corianton has his father’s assurance that the Messiah’s mission will rescue him from the grave just as it will work for those who die after the Messiah’s coming.