Rhetorical: Alma drives home his point as forcibly as possible. Alma openly accuses some in his congregation of being not of Christ, but of Satan. Of course we may assume that he does not mean every person listening to the sermon, but certainly there were a large number to warrant both Alma's resignation from the judgeship and this very pointed sermon.
It is interesting that Alma even sets up a "proof" of his contention that those wicked in the congregation are "of Satan." He suggests that if they deny it, that is in itself an indication that they are a "liar and a child of the devil." How can this be true?
One of the most common of human traits is the technique of self-preservation through rationalization. We make ourselves believe that our actions are acceptable, even when they are not. Alma knew full well that there were those in the congregation who had been able to rationalize their actions. For those who had been pierced in their hearts sufficient to repentance, they would admit their sins and begin the process of repentance. For those who continued to deny their wrong-doing, they were continuing to deceive themselves. Those people would not begin the repentance process, and thus would find themselves aligned with Satan at that last day.