How shall we read this statement that the church was more fully established and that many were baptized? We might assume that there was great missionary work, but that is not what the text says. In fact, in the context of the verses leading up to this statement, it is clear that we are looking at the effects of the repentance of those mentioned in verse 3.
Still, this brings up an important issue about Nephite society. Weren’t these people, as Nephites, already baptized? There are two possible answers to the nature of these baptisms following the great repentance. The first is that they had already been baptized, and that this was a baptism of recommitment. This type of baptism for renewal or refreshing of covenants was practiced among the Utah saints for many years, and did not begin to be officially discouraged until 1894 when the First Presidency instructed stake presidents that rebaptism was not required for temple attendance, and that repentance was the key, not the rebaptism (Allen, James B., Glen M. Leonard. The Story of the Latter-Day Saints. 1976, pp. 430-431).
While it is possible that the Nephites practiced some form of rebaptism, there is no clear record of that in the Book of Mormon. It is equally as likely that this spate of baptisms were for those of Zarahemla who were part of the land, and simply not a part of the church. As one of the reforms of Alma the Elder, the church had been separated from the state, and we began to have a distinction between those who were politically of Zarahemla and those who were baptized into the church. Since this verse specifically speaks of expanding the presence of the church, it is most likely that it is the acceptance into the church of those who were previously part of the land of Zarahemla, but had not been baptized into the church. These would be the precise people who would have had the sympathies with those who opposed the plan of God, and therefore were those who would have seen themselves as committing “wickedness and abominations.”