“Both Alma and Helam Were Buried in the Water”
There is no question but that Alma held the priesthood (see commentary on verse 18), was one ordained after the holy order of God. Thus he would have been baptized previously. “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. In Alma 5:3 we learn that Alma [the younger] was consecrated the high priest over the Church under his father. Now Alma did not organize the Church with the idea that they had no church before that time. They had a church from the days of Lehi and Alma only set things in order.” (Joseph Fielding Smith, Answers to Gospel Questions 3:203-4.)
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