Mosiah 27:16 - 27:20

Mosiah 27:16 Now I say unto thee: Go, and remember the captivity of thy fathers in the land of Helam, and in the land of Nephi; and remember how great things he has done for them; for they were in bondage, and he has delivered them. And now I say unto thee, Alma, go thy way, and seek to destroy the church no more, that their prayers may be answered, and this even if thou wilt of thyself be cast off.

The angel specifically tells Alma to keep in mind the freeing of his parents and grandparents from the land of Nephi and from the land of Helam from Lamanite domination. It was God's direct intervention that brought about their escape from the slavery they were force to live under. It was now these people that were praying with earnestness over him and that he be either brought back into the Church or he be silenced because of the havoc he and the sons of Mosiah were causing among the Church members.
[I wonder why being asked to remember the captivity of his relatives was the way to get him to stop his anti-church activities? I guess it is to remind him of the great sacrifice they made to escape Lamanite domination so they could have the freedom to worship and pursue whatever kind of life they wanted. If it had not been for their efforts to escape, Alma would not have the freedom to go about trying to destroy the Church.]

Mosiah 27:17 And now it came to pass that these were the last words which the angel spake unto Alma, and he departed.

The angel left the boys after these words of warning to them.

Mosiah 27:18 And now Alma and those that were with him fell again to the earth, for great was their astonishment; for with their own eyes they had beheld an angel of the Lord; and his voice was as thunder, which shook the earth; and they knew that there was nothing save the power of God that could shake the earth and cause it to tremble as though it would part asunder.

The effect of this angelic visit was stunning. The boys collapsed due to freight. They now knew that God was real and had the power to connect the voice of an angel with earthquakes.
[The realization in such a dramatic fashion could only brought fear and trembling to them. It’s like a man caught in the act of a crime and the fear of knowing what will happen because of it. I suspect they knew they were doing wrong deep in their souls, but they were having so much success and profiting from their efforts that it was hard not to continue. The devil had them and they were having success. A difficult combination to overcome. It takes a real scare sometimes to give the person enough power to overcome the chains of the devil.]

Mosiah 27:19 And now the astonishment of Alma was so great that he became dumb, that he could not open his mouth; yea, and he became weak, even that he could not move his hands; therefore he was taken by those that were with him, and carried helpless, even until he was laid before his father.

The effect of the angelic visit was to leave Alma without strength to move or to speak. He was paralyzed. His friends (probably the sons of Mosiah who were with him when the angel came) took Alma and laid him before his father.
[The visit did not seem to effect the others like it did Alma Jr. Maybe they were not as hard core as was Alma Jr. They had less of a guilty conscience than did Alma and the effect was not as great of them. There may have also been others in the group that saw the angel who were neither Alma Jr. or the sons of Mosiah that brought him back to town.]

Mosiah 27:20 And they rehearsed unto his father all that had happened unto them; and his father rejoiced, for he knew that it was the power of God.

The other boys told Alma Sr. what had happened to Alma Jr. and what they had witnessed. Alma Sr. was happy because he knew his prayers were answered on his son’s behave. He wanted some sort of manifestation to get him back from the dark side.
[Would be interesting to know if Alma Sr. was looking for any particular kind of wakeup call for his son. Or was he just wanting anything to get his son back to living the way had certainly had seen his father and mother had been living.]