Back in Mosiah 12:20-24, the priests of Noah asked Abinadi about the interpretation of an Isaiah passage (Isa 52:7-10). Abinadi is finally ready to give them the answer. To do it, he first had to review the Law of Moses, he had to review the reason the Law of Moses was given, he had to remind them to whom the Law of Moses pointed, he had to explain the process of Christ's atoning for sins, and he had to teach what it means to be spiritually reborn. Having done all this, he is finally ready to explain the meaning of the scripture. Thus, we see that the greatness of Isaiah's words can only be fully understood with a comprehensive understanding of God's plan. Abinadi's teachings had to be fairly comprehensive before the priests would understand the meaning of the passage.
Commentary on this Isaiah passage can be found under the section for Mosiah 12:21.