Korihor was an anti-christ. He went around preaching there was no Christ.  He said the church was telling people false things to make them follow their leaders.  He said they did this so people would pay them.  Alma said he had never taken any money to lead the church and did it because the teachings of the church make people happy.  HE also said "i have all things that prove there is a god! the earth and all things created on it!"  and he told Korihor he knew he was lying. He told Korihor if he lied one more time God would give him a sign, and he would be struck dumb (could not speak).  Korihor denied and could no longer speak.  He wrote and note to Alma and said the devil had decieved him and that he did know there was a God.  He asked Alma to pray to God to make him able to speak again.  Alma said God knew if He did that, Korihor would just go back to teaching lies to the people.  When the people heard what had happened to Korihor, those that had believed his lies, repented and came back to God and the church.