A modern reading might see this year as a period of intense missionary work, but in context, this “more fully” established church and the number of baptisms resulted from the repentance of those mentioned in verse 3.
Does this mean that the newly repentant had not previously been baptized? Possibly this baptism was an ordinance of recommitment. Such baptisms for renewal or refreshing of covenants occurred among the Utah Saints for many years. The practice was not 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.
However, there is no clear record of rebaptism as a Book of Mormon practice. It seems equally likely that many who counted themselves politically and socially as Nephites because they lived in the land of Zarahemla had simply not become part of the church until this point. One of Alma1’s reforms had been the separation of the church from the state; not all citizens of Zarahemla were baptized members of the church from that point. Since this verse specifically speaks of expanding the church, it most likely means the baptism of the previously unchurched—precisely those who had opposed Yahweh’s plan (or sympathized with those who did) and therefore would have seen themselves as committing “wickedness and abominations.”