When Ammon and his brothers decided to split up, Aaron decided to head to a place near the border of the Land of Mormon that the Lamanites called "Jerusalem" (they named it after the other Jerusalem, apparently). The Lamanites, the Amalekites, and the people of Ammon had built a big city there, and they really needed some guidance, because they were starting to get really hard hearts and doing all kinds of wickedness. Aaron had his work cut out for him.
Now when Ammon and his brethren separated themselves in the borders of the land of the Lamanites, behold Aaron took his journey towards the land which was called by the Lamanites, Jerusalem, calling it after the land of their fathers' nativity; and it was away joining the borders of Mormon. 21.1
Now the Lamanites and the Amalekites and the people of Amulon had built a great city, which was called Jerusalem. 21.2
Now the Lamanites of themselves were sufficiently hardened, but the Amalekites and the Amulonites were still harder; therefore they did cause the Lamanites that they should harden their hearts, that they should wax strong in wickedness and their abominations. 21.3Once Aaron got to Jerusalem, he started working on the Amalekites first. He started preaching at their synagogues, where they followed the Nehor doctrine. (Remember Nehor, the guy who killed Gideon in Alma 1?) The sons of Mosiah had a bit of a feud going on with the order of the Nehors. You see, the Nehor folks believed that priests should be paid for their services, and that they should be highly respected. They also disagreed about universal salvation, and whether or not a person needs to repent. (See more about Nehor here).
So when Aaron went into their synagogues, they weren't exactly receptive. They basically said: "What, have you seen an angel or something? Aren't we as good as you? Why do we have to repent? What makes you so high and mighty? We worship God, and built temples and stuff, so why can't you just leave us alone? We don't think that you actually know the things you claim to. Please leave our church."
Therefore, as Aaron entered into one of their synagogues to preach unto the people, and as he was speaking unto them, behold arose an Amalekite and began to contend with him, saying: What is that thou hast testified? Hast thou seen an angel? Why do not angels appear unto us? Behold are not this people as good as thy people? 21.5
Thou also sayest, except we repent we shall perish. How knowest thou the thought and intent of our hearts? How knowest thou that we have cause to repent? How knowest thou that we are not a righteous people? Behold, we have built sanctuaries, and we do assemble ourselves together to worship God. We do believe that God will save all men. 21.6
Now Aaron said unto him: Believest thou that the Son of God shall come to redeem mankind from their sins? 21.7
And the man said unto him: We do not believe that thou knowest any such thing. We do not believe in these foolish traditions. We do not believe that thou knowest of things to come, neither do we believe that thy fathers and also that our fathers did know concerning the things which they spake, of that which is to come. 21.8But, like a good Mormon, he didn't. He started preaching to them, in their own church, until some of the people got really angry with him, and the rest made fun of him. After a while, he realized that they weren't listening, so he decided to leave. He went to the village of Ani-Anti, and found some new people to argue with.
Now Aaron began to open the scriptures unto them concerning the coming of Christ, and also concerning the resurrection of the dead, and that there could be no redemption for mankind save it were through the death and sufferings of Christ, and the atonement of his blood. 21.9
And it came to pass as he began to expound these things unto them they were angry with him, and began to mock him; and they would not hear the words which he spake. 21.10
Therefore, when he saw that they would not hear his words, he departed out of their synagogue, and came over to a village which was called Ani-Anti, and there he found Muloki preaching the word unto them; and also Ammah and his brethren. And they contended with many about the word. 21.11But the people preaching in Ani-Anti (Muloki and Ammah) had already hardened the people's hearts, so it was useless. Aaron continued on to the land of Middoni and started preaching there. He didn't convert many people, but he was still thrown in jail, along with some of his brethren.
And it came to pass that they saw that the people would harden their hearts, therefore they departed and came over into the land of Middoni. And they did preach the word unto many, and few believed on the words which they taught. 21.12
Nevertheless, Aaron and a certain number of his brethren were taken and cast into prison. 21.13At this point, the stories merge. Ammon and Lamoni rescue Aaron and his brethren from prison, and then they split up again: Ammon and Lamoni return to the land of Ishmael, and Aaron and his posse "went forth again to declare the word". They preached to Amalekites and Lamanites all throughout the land, and they had better luck this time. They "brought many to the knowledge of the truth" and "convinced many of their sins". Nice work, Aaron!
And those who were cast into prison suffered many things, and they were delivered by the hand of Lamoni and Ammon, and they were fed and clothed. 21.14
And they went forth again to declare the word, and thus they were delivered for the first time out of prison; 21.15
And they went forth whithersoever they were led by the Spirit of the Lord, preaching the word of God in every synagogue of the Amalekites, or in every assembly of the Lamanites where they could be admitted. 21.16
And it came to pass that the Lord began to bless them, insomuch that they brought many to the knowledge of the truth; yea, they did convince many of their sins, and of the traditions of their fathers, which were not correct. 21.17
And it came to pass that Ammon and Lamoni returned from the land of Middoni to the land of Ishmael, which was the land of their inheritance. 21.18Meanwhile, Ammon and Lamoni were also making progress back in the land of Ishmael. Lamoni's father stayed true to his word, by granting Lamoni full control of his people. Lamoni rounded everyone up decreed that they could worship God in any way they wanted.
But he caused that there should be synagogues built in the land of Ishmael; and he caused that his people, or the people who were under his reign, should assemble themselves together. 21.20 And he did rejoice over them, and he did teach them many things. And he did also declare unto them that they were a people who were under him, and that they were a free people, that they were free from the oppressions of the king, his father; for that his father had granted unto him that he might reign over the people who were in the land of Ishmael, and in all the land round about. 21.21
And he also declared unto them that they might have the liberty of worshiping the Lord their God according to their desires, in whatsoever place they were in, if it were in the land which was under the reign of king Lamoni. 21.22Ammon started preaching in the synagogues there, and he taught them "all of the things concerning things pertaining to righteousness".
(This is perhaps my favorite line in the whole Book of Mormon. "And it came to pass that on Tuesday my boss taught me all of the the things concerning things pertaining to customer service...")
And Ammon did preach unto the people of king Lamoni; And it came to pass that he did teach them all things concerning things pertaining to righteousness. And he did exhort them daily, with all diligence; and they gave heed unto his word, and they were zealous for keeping the commandments of God. 21.23
That's it for Chapter 21. The next few chapters will continue to follow Aaron, Muloki, and their band of merry men as they preach to the Lamanites.