Alma will now begin a fairly extensive discourse on the saving role of the Atoning Messiah. Why does he do this? Corianton has grown up with a belief in the coming of the Atoning Messiah, but apparently he has recently abandoned that belief, a point we presume from the nature of his described sins.
When Alma needs to teach Corianton about repentance, he does not need to tell him that the Atoning Messiah will come, if Corianton is at all repentant, he understands that he must return to this belief. What Alma does need to do is make sure that Corianton understands the true process of repentance, a process that cannot be understood, indeed cannot occur, without the Atoning Messiah’s mission.