Fire is a powerful symbol in Judaism and represents the very essence of God. We see God manifest through fire everywhere in the Old Testament: The burning bush from whence God's voice came, the pillar of fire that led the Israelites out of Egypt, the fire from heaven consuming Elijah's sacrifice, etc. Fire is also a well-known and similar symbol in Christianity. After Christ's coming to earth, the Holy Ghost became a more distinct person in the Godhead and for christians (and especially Latter-Day Saints perhaps), the fire symbolism has become connected to him in particular, as we know. But in this post, I will focus on the ancient Israelite understanding.
Malachi 3:2, also quoted in 3 Nephi, states that God is like a "refiner's fire". This is a common Jewish idea. When you put metal inside a fire, what is good remains and is even purified. The unwanted material burns and leaves the dross. The fire symbolism works that way: It can represent purification on one hand or judgment and destruction on the other, depending on what "material" you are.
We see this symbolism a lot in the Book of Mormon too. Lehi also sees a pillar of fire. In Helaman 5, 3 Nephi 17 and 3 Nephi 19, there is fire from heaven encircling and purifying a group of people. (See for instance 3 Nephi 19:14 and 28). When for instance Zarahemla burns and all inhabitants in it (see 3 Nephi 9:3), a Jew could interpret that on a spiritual level to mean that the inhabitants were not able to stand God's presence because of wickedness.
The worthless leftovers from the "refiner's fire" is called dross. Reading Lord Wilmore's recent post, I came to realize that the word, 'dross', is used twice in the Book of Mormon.
In Alma 32:3
they were poor; yea, they were esteemed by their brethren asThis is a group of people who were despised by their fellow Zoramites because of their poverty, being shown the opposite of Christ-like charity. They were considered dross by them. Two chapters later, but still in the same sermon, in Alma 34:29, Amulek states
Therefore, if ye do not remember to be , ye are as dross, which the refiners do cast out, (it being of no worth) and is trodden under foot of men.Amulek's point is a good one, but even better when we consider the reference to Alma 32:3. Amulek is saying that poverty doesn't make dross, as the rich Zoramites claimed, but rather the lack of charity and those who don't " of [their] substance" (Alma 34:28). In other words, those who considered their bretheren "dross" in Alma 32:3 are actually dross themselves. Further, it is a warning to the poor among the Zoramites and everyone else, that if we are not charitable, we become dross just like the rich Zoramites and will be consumed or cast out when God appears.
This also gives some context to Moroni 7:47-48
If we are filled with charity, there will be no dross when the fire appears. Instead, we will be purified.