By the time Alma spoke to his people in chapter 5, the people of Zarahemla had strayed quite a bit, as evidenced by Alma’s later speech to the people of Gideon. In Alma 7:3–5 he compared the righteousness of the people of Gideon to "the awful dilemma that our brethren were in at Zarahemla." In Alma 4, he spoke of contentions, pride, and wickedness entering into the church, and in chapter 5, he told the people of Zarahemla to repent.
In verse 21, he said, "Ye cannot be saved, except your garments are washed white," so presumably, not all their garments were white yet. In verse 23, he called at least some of them murderers, and said they were guilty of all manner of wickedness.
It seems as though there were a lot of people sitting on the fence in Zarahemla. The population was not as cohesive as it had been 42 years earlier under Benjamin, and Alma sought to get back to that kind of unity, to find some way to get people all back together again.