A major theme from the Book of Mormon summarized in D&C 29:42-44

I enjoy finding succinct summaries of major themes of the Book of Mormon in other books of scripture, especially the Doctrine & Covenants. Here is one example:

42 But, behold, I say unto you that I, the Lord God, gave unto Adam and unto his seed, that they should not die as to the temporal death, until I, the Lord God, should send forth angels to declare unto them repentance and redemption, through faith on the name of mine Only Begotten Son.

43 And thus did I, the Lord God, appoint unto man the days of his probation—that by his natural death he might be raised in immortality unto eternal life, even as many as would believe;

44 And they that believe not unto eternal damnation; for they cannot be redeemed from their spiritual fall, because they repent not; (D&C 29)

Angels declaring repentance show up all over the Book of Mormon, actually from its very first chapter to its very last. In some cases, the angels are heavenly messengers, which is how we commonly think of angels.  But the word simply means 'messenger' and the Book of Mormon shows us over and over again how humans can hearken to the word and become angels delivering the message of repentance. 

Deep doctrine taught plainly

The verses from D&C 29 above teach a very deep point of doctrine: The fall of Adam brought us into mortality and made inevitable a future day of judgment.  We are also taught that angels will be sent forth declaring repentance prior to that day of judgment. Everything that we experience between now and our day of judgment can/should be thought of as "today" --

23 Behold, now it is called today until the coming of the Son of Man, and verily it is a day of sacrifice, and a day for the tithing of my people; for he that is tithed shall not be burned at his coming.

24 For after today cometh the burning—this is speaking after the manner of the Lord—for verily I say, tomorrow all the proud and they that do wickedly shall be as stubble; and I will burn them up, for I am the Lord of Hosts; and I will not spare any that remain in Babylon.

25 Wherefore, if ye believe me, ye will labor while it is called today. (D&C 64)

Deeper doctrine taught a little less plainly 

Years of thinking and studying about the role angels play in mortality have led me to understand that it is God's intention to turn each of us into a ministering angel.  Participation in the everlasting covenant requires this. Many scriptures teach this point, but do so a little more subtly, as in verse 25 of section 64 above. I've posted many times about this (see here).  See also D&C 88:77-81.

The "labor" which is to be performed is mediated by God's word and God's messengers, also known as "Cherubims and a flaming sword" (see Genesis 3:24). In D&C 88:74-75 the work is defined in detail:

"[God's] work" defined:
"I give unto you ... a commandment that you:

• assemble yourselves together, and 
• organize yourselves, and 
• prepare yourselves, and 
• sanctify yourselves; yea, 
• purify your hearts, and 
• cleanse your hands and your feet before me, 

that I may:

• make you clean; ...
• testify unto your Father, and your God, and my God, that you are clean from the blood of this wicked generation; ...
• fulfil this promise, this great and last promise, which I have made unto you, when I will.

The cleansing of the hands and feet makes reference to joining in the work. Thus, it seems that an important component of the process of sanctification is our willingness to simultaneously enter into God's rest and labor in the vineyard. I love the symmetry there.  It seems Mormon did, too.  Notice this subtle description of Alma-2 near the end of his time among the Nephites:

1 And now it came to pass that the sons of Alma did go forth among the people, to declare the word unto them. And Alma, also, himself, could not rest, and he also went forth.
2 Now we shall say no more concerning their preaching, except that they preached the word, and the truth, according to the spirit of prophecy and revelation; and they preached after the holy order of God by which they were called. (Alma 43)

I love it.  Alma taught about the word, the "holy order of God" and entering into God's rest in his sermon found in Alma 12-13. This note from Mormon ties all of those concepts together and relates them to his specific actions. Having long since entered into God's rest, Alma, though he was old, could not watch his sons go off to preach the word among the people without joining them.