This is the great mystery that Alma reveals! It seems so simple to us, why was this such a mystery to Alma that he gave it such a build up? Once again, we must remember the differing perspectives of our timeframes. We are living in a world that followed the mission of Christ, Alma and Corianton were living in a very different world. Remember that the introduction to this important announcement dealt with the temporal disjunction between the people living before and after the mission of Christ.
The reason that the timeframe is at issue is that Christ will do something during his mission. He will effect an atonement. It is very easy to understand that the atonement is effective after it has taken place. It is much more difficult to understand how it might be effective before it has taken place. How will those who live before the atonement occurs receive the benefit of something that hasn’t yet happened?
This is the reason that the marvelous answer that Alma gives is that all shall come forth from the dead. This is not an abstract theological issue, but one that is absolutely pertinent to Alma and Corianton. As men living prior to the mission of the Christ, the resurrection, this promise of incorruption, will not happen for them. That happens only after the coming of the Christ. Nevertheless, not only those who die after Christ’s mission will be resurrected, but all will be resurrected. What Alma is saying to Corianton is that Corianton can receive the benefit of the resurrection, even though he lives prior to the time when the act will be completed that allows the resurrection. All that is different is the timing. The blessing will come, it will just be delayed until after the time of the Atoning Messiah.