First, a list of promises in these chapters:
Prophecies of Isaiah shall be fulfilled
Record of the Lehites shall go forth to the Gentiles
Whoever hearkens to the Lord, repents and is baptized shall be saved.
A messenger will prepare the way before the Lord.
The Lord shall come suddenly to His temple (fulfilled at Kirtland temple, but does this also refer to Second Coming?)
The Lord will purify the sons of Levi.
They will offer Him an acceptable offering in righteousness.
The Lord will witness against iniquity and oppression
He will not change and can be relied upon
The Lord returns to us when we return to him
He will pour out blessings beyond measure on tithe payers, including:
rebuking the devourer to protect their righteous endeavors,
helping the fruits of their labours come forth at the appropriate time,
blessing them in the eyes of all nations,
make their land delightsome
The Lord hears, notes and remembers their concerns and the thoughts of their hearts
Those who honor Him will be His treasure
He will spare them as His own children and they will see the blessings of serving Him.
The glory of the Lord that consumes the wicked at His coming will bring healing to those that honor Him
It seems that in the last couple of chapters, the Lord gave us the big picture of what He was going to do for Israel. This is only one of many interpretations, but what I see right now in these chapters is a more detailed outline of the process for purifying individuals.
First, there's a reassurance that the big picture will come to pass. Then, our personal purification begins with the coming forth of the Book of Mormon and the Restoration. For me, the Book of Mormon is vital; it's my personal Liahona. Over and over, the Lord has given me understanding, course corrections and reassurance as I study it.
Then, we have the restoration of keys and a manifestation of the Lord at the Kirtland Temple and a promise to purify the sons of Levi. This probably has particular application to the literal descendants of Levi and their being brought to Jesus Christ. But I believe there are layers of meaning here, including the purifying of all His priests. As this promise immediately follows a reference to His coming to His temple, I'm inclined to give the most generous possible interpretation to who is to be purified and offer an offering in righteousness. I believe it includes me.
Purification begins as the Lord witnesses to me against my iniquity and the oppressions I commit. I learn to trust Him and to return to Him and to delight in relationship with Him.
Then we have the promises associated with the tithe. We obey the law and the Lord pours out blessings in such abundance that we don't have room to receive them. To me, this seems to describe a common early phenomenon of discipleship -- we see visible and often tangible evidence that obedience to the gospel improves our lives, richly.
But there's further refining ahead. 3 Nephi 24:13-15 addresses a different phenomenon; when we seem to suffer for our faithfulness, in contrast to those who disobey and seem to be rewarded by the world. When we are rejected, passed over, persecuted, and afflicted for living consistent with our faith, it's easy to think something is wrong, that "it is vain to serve God." If wickedness never was happiness, then why are the proud and wicked apparently happy, while we walk mournfully? This is a legitimate question and the cross references direct to such feelings by three prophets, Job, Jeremiah and Habakkuk..
In that context, I love the Lord's explanation. He who is outside of time says, "Just wait. The wicked may seem 'set up' and the proud happy, but their well-being will not last and you who serve me in sunshine and rain, joy and tears, are MINE, even my jewels."
I'm reminded of the beatitude, "Blessed are ye when men shall revile you and persecute, and shall say all mannner of evil against you falsely, for my sake ... For ye shall have great joy and be exceedingly glad... for so persecuted they the prophets who were before you" (3 Nephi 12:11,12).
Why would the Lord invite us to endure persecution for righteousness sake, and even to rejoice at the opportunity?
In Robert Bolt's historical play "A Man for All Seasons," Sir Thomas More is imprisoned and in peril of his life on a point of conscience when his daughter, Margaret, accuses him of self-importantly playing the martyr. He replies, "If we lived in a State where virtue was profitable, common sense would make us good, and greed would make us saintly. But since we see that avarice, anger, pride and stupidity commonly profit far beyond charity, modesty, justice and thought, perhaps we must stand fast a little... even at the risk of being heroes."
"But in reason!" Margaret protests. "Haven't you done as much as God can reasonably want?"
"Well, finally it isn't a matter of reason," More replies. "Finally, it's a matter of love."
And so it is. The truth is that when we serve God while He prospers us, we don't really know if we serve Him for love of Him, or because of the blessings He gives us. But when we serve at the apparent cost of our other desires, when our hearts are ripped out, our hopes are dashed, and we carry on, clumsy and stumbling and feeling abandoned but pushing forward through the dark because He is the greatest good we know, He becomes our hope, our heart and our treasure. And we become fully His.