The people were willing and excited to accept this new covenant, and baptism became the symbol of that covenant. To begin, Alma selects Helam and takes him down into the water. After invoking authority from God, Alma and Helam are both immersed.
At this point in Nephite history, the use of immersion in baptism is clear, but it is an immersion borrowing significance from the previous Mikveh rite of cleansing, which was by immersion. Later Christianity used the immersion as a symbol of death and resurrection, but that symbolism was only possible after Christ’s death and resurrection. That imagery was unavailable at this time, and the Book of Mormon will not use that symbolic association with baptism.
When Alma baptized with authority, with what authority did he baptize? There will later be a discussion of authority, but the important point here is that Israel had two means of assigning religious authority to perform religious acts. One was lineal, and we know it as the Levitical Priesthood. That was not the only way, however. A king had the divine right and the ability to provide authority. Alma had such authority from Noah. Although we know Noah as a bad king, that does not mean that actions performed with good faith through his granted authority are not divinely recognized. Even today, we understand that the act can be valid in God’s eyes even if, or when, the one performing the act is not fully worthy. We are blessed for our action and beliefs, not those of another.
Nevertheless, the most important indication will come in verse 18. Alma acted, “having authority from God.” If God authorizes Alma, we need not worry about the intermediaries.