Rhetorical: Abinadi now brings his discourse to the point of answering the question asked of him. He answers the question at this late date by expanding on Isaiah 52:7/Mosiah 12:21. He recites and expands the theme of that verse so that it is clear that he is answering the question, but doing so in the light of the rest of his discourse. The original verse cited is:
Mosiah 12:21-24 (Isaiah 52:7-10)
21 How beautiful upon the mountains are the feet of him that bringeth good tidings; that publisheth peace; that bringeth good tidings of good; that publisheth salvation; that saith unto Zion, Thy God reigneth;
Abinadi's connection between his discourse and this verse rests upon the idea that the "feet" represents a person (using the idea of a part representing the whole); that Isaiah's trilogy of publishing peace, good tidings and salvation are all terms for the same thing, and that this peace/good tidings/salvation is the atonement through the Messiah who is to come.
It is particularly important that he makes sure that he cites the end of Isaiah 52:7 (Mosiah 12:21) which is "Thy God reigneth." Since Abinadi's point as been that God himself should come down to provide the atonement for man, he interprets Isaiah's declaration as directly referring to this Messiah. For Abinadi, this is a direct declaration of the Messiah as God, and the triumph of this God/atoning Messiah. It is precisely the point he wants to make to counter the priests. Not only do they not keep the law of Moses because they do not teach of the coming atoning Messiah, but they do not even understand the verses they have cited, for they declare that atoning Messiah directly! While we can read "Thy God reigneth" as God the Father, that would weaken the whole of Abinadi's discourse and logic. All of his rhetorical direction has led to this point where he can add his prophetic declaration to Isaiah's, and declare the atoning Messiah as God himself (see Mosiah 15:1, with comments).