This verse depends upon the last sentence of verse 11. That sentence focuses on Alma the Elder believing the words of Abinadi. In this verse Alma the Younger suggests that the belief of his father in Abinadi wrought a "might change" in Alma the Elder's heart. Doubtless the current congregation remembers the story of Alma the Elder, and would understand just how great this "mighty change" was. Alma the Elder was a priest of Noah, and as a priest of Noah would have been active in preaching against the word of God in the same way as the priests questioned Abinadi. Alma the Elder would have been a supporter and at least a spiritual enforcer of what Alma the Younger has described as the "chains of hell."
To move from a supporter of the opposition to a leader of the people of God indeed required a mighty change of heart. At the end of this, we have Alma the Younger's affirmation that "behold I say unto you that this is all true." This declaration of testimony is all the stronger because Alma the Younger is not just alluding to his father, but to his own mighty change. He knows firsthand what is required to change from a persecutor to a leader of the people of God. His audience would also certainly know his personal story, and they would not have missed this double intent of Alma the Younger's declaration of truth.