So verses 21-24 are quoting Isaiah 52:7-10. As I have showed before, Abinadi comes back to themes and expressions from this passage all throughout his sermon. We know that Alma was the only one who believed in Abinadi and wrote down his words and preached them. Lord Wilmore has demonstrated how the role of Abinadi as doctrinal mentor for Alma influenced him and even his son, Alma the younger (like here and here). Therefore it is also interesting to note how Mormon in his abridgment of the early stages of both Alma-1 and Alma-2's conversions, alludes to this Isaiah passage.20 And it came to pass that one of them said unto him: meaneth the words which are written, and which have been taught by our fathers, saying:21 beautiful upon the mountains are the feet of him bringeth good tidings; that publisheth peace; that bringeth good tidings of good; that publisheth salvation; that saith unto Zion, Thy God reigneth;22 watchmen shall lift up the voice; with the voice together shall they sing; for they shall see eye to eye when the Lord shall bring again Zion;23 Break forth into joy; sing together ye waste places of Jerusalem; for the Lord hath comforted his people, he hath redeemed Jerusalem;24 The Lord hath made bare his holy in the eyes of all the nations, and all the ends of the earth shall see the salvation of our God? (Mosiah 12)
First in Mosiah 18:30
And now it came to pass that all this was done in Mormon, yea, by the , in the forest that was near the waters of Mormon; yea, the place of Mormon, the waters of Mormon, the forest of Mormon, how beautiful are they to the eyes of them who there came to the knowledge of their Redeemer; yea, and how blessed are they, for they shall to his praise forever.The color coded connections may not seem significant. But notice the "how blessed are they" expression again in conjunction with Alma-2's conversion and the unmistakable reference to Isaiah 52.
36 And thus they were instruments in the hands of God in bringing many to the knowledge of the truth, yea, to the knowledge of their Redeemer.
37 And how blessed are they! For they did ; they did publish good tidings of good; and they did declare unto the people that the Lord reigneth. (Mosiah 27:37)There is only one other place where this expression shows up in the Book of Mormon, in 4 Nephi 1:17-18
There is no Isaiah quote here at all, only perhaps a description of a time like the one Isaiah was prophesying about. But prior to this, as Jesus is visiting the Nephites, there are more Isaiah 52:7-10 quotes as he speaks of the latter-day gathering of Israel. Once in 3 Nephi 1617 There were no robbers, nor murderers, neither were there Lamanites, nor any manner of -ites; but they were in , the children of Christ, and heirs to the kingdom of God.18 And how blessed were they! For the Lord did bless them in all their doings; yea, even they were blessed and prospered until an hundred and ten years had passed away
...and once in 3 Nephi 20. Here he quotes almost all of Isaiah 52, but in a different order, see this post for details. Verses 7-10 are quoted as follows17 And then the of the prophet Isaiah shall be fulfilled, which say:18 shall lift up the voice; with the voice together shall they sing, for they shall see eye to eye when the Lord shall bring again Zion.19 Break forth into joy, sing together, ye waste places of Jerusalem; for the Lord hath comforted his people, he hath redeemed Jerusalem.20 The Lord hath made bare his holy arm in the eyes of all the nations; and all the ends of the earth shall see the salvation of God.
I think it's interesting to note that "the salvation of God" is replaced by "the salvation of the Father" with the addition "and the Father and I are one" in verse 35.32 Then shall their lift up their voice, and with the voice together shall they sing; for they shall see eye to eye (Isaiah 52:8).33 Then will the Father gather them together again, and give unto them for the of their inheritance.34 Then shall they break forth into joy— together, ye waste places of Jerusalem; for the Father hath comforted his people, he hath redeemed Jerusalem (Isaiah 52:9).35 The Father hath made bare his holy arm in the eyes of all the nations; and all the ends of the earth shall see the salvation of the Father(Isaiah 52:10); and the Father and I are one...40 And then shall they say: beautiful upon the mountains are the feet of him that bringeth good tidings unto them, that peace; that bringeth good tidings unto them of good, that publisheth salvation; that saith unto Zion: Thy God reigneth! (Isaiah 52:7)
Why does Mormon use the "how blessed were they" expression when referring to Isaiah 52 at least twice? Isaiah 52 says nothing about being blessed. The answer may be found in 3 Nephi 20 just quoted above. A few verses prior to this, the Savior states:
25 And behold, ye are the of the prophets; and ye are of the house of Israel; and ye are of the which the Father made with your fathers, saying unto Abraham: And thy seed shall all the kindreds of the earth be blessed.
26 The Father having raised me up unto you first, and sent me to you in away every one of you from his iniquities; and this because ye are the children of the covenant—Before quoting Isaiah 52, Jesus quotes the promise to Abraham given in Genesis 12:3. The blessing associated with Isaiah 52 is a consequence of being part of a covenant people. Since 3 Nephi 20 probably is a direct quote from the large plates of Nephi, it's easy for me to imagine Mormon studying this, making the connection between the Abrahamic covenant and Isaiah 52 and applying this when writing about Alma the elder and younger. After all, Alma-1 and his followers truly were blessed by entering into the covenant through their baptism at the Waters of Mormon.
