Mosiah 13:28

"And moreover, I say unto you, that salvation doth not come by the law alone; and were it not for the atonement, which God himself shall make for the sins and iniquities of his people, that they must unavoidably perish, notwithstanding the law of Moses."

Jesus Christ was prophesied about long before He came to the earth.  Abinadi is teaching the people that Christ will come and atone for the sins of the people so that they can repent and be saved in the Kingdom of God.  The people lived the law of Moses thinking that it was enough, but Abinadi teaches that the law leads to the atonement.  Christ fulfills the law of Moses with his atonement.