Biographical: We are now introduced to Alma, who will become one of the most important figures in the Book of Mormon. We don't have much information about who Alma is. This verse tells us:
The most perplexing comment is that "he was a young man." According to the chronology worked out and discussed after Mosiah 7:1, we have Alma being born in the year 159 BC, and dying at the age of 82 in the year 77 BC (probable dating, though the time depth is explicit). According to the correlation of dates, Mosiah I leaves the City of Nephi in 148 BC, and Zeniff leaves Zarahemla in approximately 143 BC. Thus when Mormon states that Alma is a descendant of Nephi, he may mean a literal descendant from both the lineage and the city. Alma the Elder would have been 11 when Mosiah I left the City of Nephi, and would have returned with Zeniff when he was 16. While that is young enough, he would have been 36 when Zeniff died and Noah took power. Abinadi is clearly coming later, perhaps between five and ten years later. At 40-45 years old, it is hard to see how Mormon could call Alma "young." It is quite possible that Mormon never bothered to work out the dates, and his sources never said. We may be seeing a presumption on Mormon's part that was simply mistaken.
Somewhere, however, Mormon is getting information that could not have been in the texts concerning this immediate story. The knowledge that Alma is a descendant of Nephi presumes some genealogical information that is not stated. It is not absolutely clear how Mormon means this genealogy. Is Alma of the direct lineage of Nephi, or simply a Nephite? The first meaning requires a very specific genealogy, which Mormon never acknowledges, but could certainly have been on some record (probably from the Zarahemla period). However, it is also possible that the second meaning was the intent, to indicate that Alma was a lineal Nephite rather than one of the people from Zarahemla, or even perhaps descended from the "others" who would have entered into the Nephite culture before this time.
That Alma was "one among them" is clear. He hears Abinadi. We may suppose that Alma is one of the priests who are confronting Alma. This would mean that he was not one of the priests of Zeniff, but was a follower of Noah's reforms (apostasy). Alma required conversion, and Abinadi converted Alma through the power of his discourse that reached Alma even though it did not reach any other priest. Perhaps Alma the Elder's patience with his own son later in life had roots in his own conversion experience. He would clearly understand how one might be deceived by tempting cultural changes that would lead to religious change. He would also clearly understand how one could be turned completely by an experience with the power of God. He would understand his son better than his son would know.
Abinadi comes to the city of Lehi-Nephi to preach. He appears to have little impact on the people, though perhaps he was preparing those who eventually follow Alma out to the wilderness. Abinadi comes before the court of Noah, and preaches so powerfully that they are unable to lay a hand on him. Why is this message so important that it must be delivered? The message had no impact on Noah. The message had an impact on only one person, Alma. Alma may have been in a state of apostasy at the time, but Alma would prove to be one of the most important figures of the Book of Mormon, in ways we shall see as his story continues to develop. It appears that Abinadi was sent on a mission that spelled certain physical doom for himself because God needed to touch one man; Alma. Is there any more powerful case for God's concern with an individual?
Mosiah 17:3
3 But the king was more wroth, and caused that Alma should be cast out from among them, and sent his servants after him that they might slay him.
Mosiah 17:4
4 But he fled from before them and hid himself that they found him not. And he being concealed for many days did write all the words which Abinadi had spoken.
By pleading for Abinadi, Alma is seen to be taking Abinadi's perspective, and is therefore now an enemy to the king, just as was Abinadi. The first command from Noah is to banish Alma for their midst. It is a second command that others will follow Alma to kill him. We are not told why Noah first banishes, and then sends men to kill, rather than having Alma killed while he was nearby.
Textual: Alma finds a way to avoid those who searched for him. While in hiding, he writes Abinadi's words. It is very probable that the record we have of Abinadi before Noah owes its source to Alma, particularly the very positive statements about the effect Abinadi has upon the court, showing forth the power of God. This information could easily have been in Alma's account, but would have been less likely in the official court account.
Nevertheless, Mormon is using more than one account here. This is evidenced by his ability to pick up the story immediately after the banishment of Alma. With Alma gone, the narrative can no longer depend upon Alma's memory. Thus we have a minimum of two sources for the Abinadi story, Alma and Noah's court records. The citations from Abinadi likely came from Alma, and the break from citation to narrative probably also marks a shift in source for Mormon. He cited Alma's account of Abinadi, and returns to summarizing Noah's official records, as he had done before.