When we read that “when Alma had heard these words he wrote them down that he might have them, and that he might judge the people of that church according to the commandments of God,” we should understand that this was more than writing something down so that Alma wouldn’t forget. This was the creation of new scripture, a new rule for how the church was to operate and how the high priest was to make judgments.
The rest of the verses indicate that, just as we expect of Jehovah’s faithful servant, Alma did as he was instructed. Those who repented were numbered among the church. Those who did not had their names blotted out. Of course, those who were unrepentant had already left the church as a belief system. The blotting of names was the more formal indication that the judgment had been made that they had rejected their covenant with Jehovah as the coming Messiah and true representative of the law of Moses.