"And they all cried aloud with one voice." How did this happen? This was most likely a coronation-affirmation or covenant-making ritual, and the people may well have known what they were supposed to say in this situation. That does not mean that they were not sincere in what they were doing. This was not just some kind of prepared, mechanical ritual. For example, when we go to a temple dedication, we do the Hosanna Shout, and we all know what we are supposed to say, but since you know that you only say or do that under certain circumstances it has a lot of meaning for us.
It was a similar thing among King Benjamin’s people. When they were saying, "Oh have mercy and apply the atoning blood of Christ," they were saying, in effect, "We want that blood to be sprinkled on us and to purify us," just as the High Priest traditionally would make the all-important atoning sacrifice on the Day of Atonement and then sprinkle purifying blood on the altar of the temple. They had just been told that the only way to overcome the natural man is through the atoning blood of Christ, so they especially wanted that blood applied to them. A similar covenant-making episode and vocal response from the people is found when Joshua and his people made a covenant with the Lord (Joshua 24:16, 21–26).
As was mentioned in connection with Mosiah 2:5 in the previous week’s discussion, the fact that each family was sitting in their tent with the door open to the temple, links Benjamin’s speech with the Feast of Tabernacles. That was the traditional time for the coronation of kings in ancient Israel. In addition, the Day of Atonement, which was the highest and holiest day of the year on the ancient Israelite calendar, came on the tenth day of the seventh month, with the week of tabernacles following right afterwards, ending with a day of great joy and rejoicing for the teaching (torah) of the Lord.
Book of Mormon Central, "How the Law of Moses Teaches about the Atonement (Alma 34:14)," KnoWhy 424 (April 12, 2018).
Hugh W. Nibley, "Assembly and Atonement," King Benjamin’s Speech: "That Ye May Learn Wisdom", ed. John W. Welch and Stephen D. Ricks (Provo, UT: FARMS, 1998), 119–145.
Terrence L. Szink and John W. Welch, "King Benjamin’s Speech in the Context of Ancient Israelite Festivals," in King Benjamin’s Speech: "That Ye May Learn Wisdom", ed. John W. Welch and Stephen D. Ricks (Provo, UT: FARMS, 1998), 147–223.