Our ability to understand the plan of redemption offered by the atonement is contingent on our faith and repentance. These two requirements are not throwaway religious formulae. They really are requirements for understanding God’s plan.
First, without faith we do not ask for more information nor are we motivated act upon the information we receive. Without faith in God, the plan of redemption would be a curiosity, similar to any other creation myth from another people.
Second, repentance humbles us, opens our soul to receive the Spirit, and allows us to be accepting. If we hear God’s word and remain unrepentant, the message does us no good. This is Alma’s point. If the Ammonihahites remain unrepentant, then his preaching will do them no good and their doom will inevitably fall upon them. (See “Excursus: The Principle of Repentance,” following 3 Nephi 18.)