Alma declares his authority for these statements. It was important that he do so, for he was condemning nearly all in his audience. With such a potentially condemnatory sermon, who was he to pronounce it? Alma declared that he did so under his authority in the Church. In the broad sense, we would call it the priesthood, for he invoked the holy order of God. It is possible, however, that he intended it to be more than simply the priesthood. There were ordained priests, but they did not have the authority to judge the congregation. Alma did through his ordination to be the high priest and head authority over the church.
The next unasked, but important, question is how Alma knew this information. Therefore, he asks: “do ye not suppose that I know of these things myself?” The similarly unstated answer had to be that they believed that he did. Then the logical question was: “and how do ye suppose that I know of their surety?” He tells them in the next verses.