Social: This is an important verse for the development of Nephite religion. As a background, we must remember that Limhi is the son of Noah who was the son of Zeniff, who was a Nephite who had left the city of Nephi with Mosiah I and come to Zarahemla before leading his people back to the land of Nephi. Zeniff was a Nephite, and Mormon views him as a "good" Nephite, which for Mormon, means one who follows the commandments of God. While Noah departed from this way, Limhi was apparently still a faithful man.
All of this is a prelude to this sermon by Alma before Limhi's people. These were people who were of the Nephite religion, and while they may have temporarily adopted some of Noah's apostatate ideas, they had discarded them by the time of their flight to Zarahemla. Thus we have the situation where Nephites who had know of baptism since the time of Nephi I are now being baptized.
If we interpret this in modern terms, they are being rebaptized after their falling away. In the more ancient context, however, it is more probable that Alma is introducing baptism in a new light and new meaning. For Alma, baptism has (for the first time) become symbolic of a covenant rather than a cleansing. Rather than an individual action looking solely to the remission of sins on a personal level, the baptism of Alma also introduces the one baptized into a congregation.
As noted before in this commentary, ancient religion was part of the entire community. Alma creates a method of separating elements of society into a different type of religious covenant that could be separate from the political covenant that binds them together. Alma creates "churches."