In these verses, Alma encourages these people to come and be baptized unto repentance, that they can be washed of their sins, that they can witness to God that they will keep his commandments and basically enter into the entire baptismal covenant that his father Alma had instituted at the Waters of Mormon.
They are probably going to be baptized in a river or natural body of water. We don’t know where this could have taken place, but anywhere in the Western Hemisphere in Alma’s day, there could have been danger in these waters. These converts could have encountered alligators, water moccasins, slippery banks, or river currents. I doubt that many of these people could swim. Lakes and springs were thought, in most ancient civilizations, to be openings into the underworld. I remember being in a small boat on the Usumacinta River in southern Mexico looking at huge alligators on the sandy beach as we went by, and I sat very still in the middle of the boat and hoped that it didn’t spring a leak! Going down into the water might well have been fearsome to these people, and so being baptized was a serious matter.
In the ancient Near Eastern world, in fact, if you were challenged as an accuser or as a witness in court, and there were conflicting testimonies or a lack of decisive evidence, so that the judges couldn’t really tell who was right or wrong, the river ordeal was the way in which Babylonian law resolved the matter. In such a case, one of the people—the one who seemed to be the weakest as a witness—would be required to submit to the river ordeal which consisted of being taken out in the middle of the river and thrown in. If the person was able to get to shore, this was a sign that the gods had favored you and your testimony would then be believed. But if you were a false witness telling a lie, you would be swallowed up and swept away by the wild river.
The people in Gideon were surely still of mixed faithfulness. Undergoing baptism would show and strengthen their spiritual determination and would be a strong testimony, witnessing to God, that they were telling the truth as they said that they wanted to enter into a covenant and be true and faithful in keeping the commandments. But at the same time in that world, once you were baptized, it made sense, symbolically and legalistically, that you owed your allegiance to God, who had sustained you through this test. That allegiance would be owed to the church which Alma was establishing, regardless of the consequences. And in Alma’s world, being a member of the church was not a casual matter. Besides alligators and the wildness of the river, persecutions of believers was intense in places like Ammonihah. I don’t think we should downplay the word "fear," when Alma tells these people to "come and fear not, and lay aside every sin, … which doth bind you down to destruction" (7:15).