Alma the younger is referring to the conversion of his father. Alma the elder doesn’t tell us much about the mighty change which transformed him from a wicked priest of Noah to the sole supporter and convert of Abinadi. The record states that after he was thrown off the court, he hid himself for many days [and] did write all the words which Abinadi had spoken (Mosiah 17:4). And Alma…repented of his sins and iniquities (Mosiah 18:1). This must have been the time of his personal Gesthemane in which he suffered until he had his mighty change of heart.
“A Mighty Change Was Also Wrought in Their Hearts”
Joseph F. Smith
“What a glorious condition was this! a condition in which the Lord God Omnipotent, by the power of His Spirit, had wrought a mighty change in the hearts of that people, that they no longer had any desire to do evil, but were filled only with a fervent desire to do that which was good. This was indeed a great change, and yet it is precisely that change that comes today to every son and daughter of God who repents of his or her sins, who humble themselves before the Lord, and who seek forgiveness and remission of sin by baptism by immersion, by one having authority to administer this sacred ordinance of the Gospel of Jesus Christ. For it is this new birth that was spoken of by Christ to Nicodemus as absolutely essential that men might see the Kingdom of God, and without which no man could enter into the Kingdom. Each of us can remember, perhaps, the change that came into our hearts when we were baptized for the remission of our sins…The feeling that came upon me was that of pure peace, or love and of light. I felt in my soul that if I had sinned-and surely I was not without sin-that it had been forgiven me; that I was indeed cleansed from sin; my heart was touched and I felt that I would not injure the smallest insect beneath my feet. I felt as though I wanted to do good everywhere to everybody and to everything. I felt a newness of life, a newness of desire to do that which was right. There was not one particle of desire for evil left in my soul. I was but a little boy, it is true, when I was baptized; but this was the influence that came upon me, and I know that it was from God, and was and ever has been a living witness to me of my acceptance of the Lord.” (Conference Reports, Apr. 1898, p. 65)
Neal A. Maxwell
“Disciples are to make for themselves ‘a new heart’ by undergoing a ‘mighty change’ of heart (Ezekiel 18:31; Alma 5:12–14). Yet we cannot make such ‘a new heart’ while nursing old grievances. Just as civil wars lend themselves to the passionate preservation of ancient grievances, so civil wars within the individual soul-between the natural and the potential man-keep alive old slights and perceived injustices, except in the meek.” (Meek and Lowly, p. 55)