These verses end the sermon itself. Alma issues the call to repentance by noting that they should “prepare … the way of the Lord.” That is a reference to the coming atoning mission of the Messiah and is the teaching of that which is to come which Alma had mentioned in verse 20.
As Alma finishes, he declares that he speaks by authority of an angel of God. He minces no words, declaring that they have hardened their hearts against the word of God and are a lost and fallen people. Those conditions remove them from Jehovah’s covenant protection. Although he does not restate it here, these are the conditions that will lead to the destruction of the city.