The ability to know about the plan of redemption (or the Atonement) is contingent upon our faith and repentance. These two requirements are not simple throwaway religious language. They really are requirements for understanding the plan of God.
First, without faith we do not ask for more information, and we do not have the motivation to act upon the information we receive. Without faith in God, the plan of redemption would be a curiosity, similar to creation mythologies from various peoples all over the world.
Second, repentance allows us to be of an accepting mind. Repentance humbles us and opens our soul to receive the spirit. If we hear the word of God and remain unrepentant, it does us no good. This is the thrust of Alma's message to the Ammonihahites. If they remain unrepentant, then his preaching will do them no good, and their destruction will be assured.