The previous verses introduced readers to Alma the younger as well as the sons of Mosiah. The fact that Alma was identified by name, but the sons of Mosiah remained generic, signified that this story is about Alma. That story begins in earnest in these verses.
Alma and the sons of Mosiah are traveling together, and are involved in some form of action that Mormon suggests is seeking to destroy the church. Since we have the word church here, it is probable that they are attempting to return to the earlier way of life, prior to Alma the elder’s establishment of churches.
Prescient of what would happen to Saul on the road to Damascus, an angel appears to the brothers. The actual conditions differ in the physical surroundings. There is a voice of thunder and a shaking of the earth. There are more references to Jehovah as a God of the earth in the Book of Mormon than remain in the Bible, and the voice of thunder and shaking of the earth are designed to tell Alma and the sons of Mosiah that the angel really is appearing with Jehovah’s authority. The experience overwhelms them, and all fall to the ground.