3 Ne. 19:10-12
“Alma was baptized and held the priesthood before the coming of Abinadi, but he became involved with other priests under the reign of the wicked King Noah, and when he baptized Helam, he felt he needed a cleansing himself so he buried himself in the water as a token of full repentance.” (Joseph Fielding Smith, Doctrines of Salvation, 2:336-337)
“We may conclude that Alma held the priesthood before he, with others, became disturbed with King Noah… . If he had authority to baptize that is evidence that he had been baptized. Therefore, when Alma baptized himself with Helam that was not a case of Alma baptizing himself, but merely as a token to the Lord of his humility and full repentance… . If I remember correctly, there is no reference to the baptism of Alma the elder or Helaman nor of Nephi and his brother Jacob, but we know they were baptized as were all the faithful members in the Church.” (Joseph Fielding Smith, Answers to Gospel Questions, 3:203-204)