What does Malachi 3:3 mean?
The first accusation brought in Malachi was targeted at the priesthood, for offering impure sacrifices (Malachi 1:6–8). As a result, God threatened to remove impurities from His sanctuary in the same way that priests discarded animal wastes (Malachi 2:3). Here, the same purification is in mind. The priesthood will be "refined."A "refiner's fire" is the process used to remove impurities from precious metals. Since the heat required to melt silver and gold is enough to destroy other materials, this is the method used to generate purer alloys. Here, that metaphor is being applied to the priesthood of Israel. The tribe of Levi was charged with caring for the temple, and only Levites could be priests. Rather than abandon them (Malachi 1:2–5), or destroy them (Malachi 3:6), God plans to refine them. Of course, this means passing through the fire.
Earlier, it was said that God would prefer no sacrifice at all, as opposed to offerings which were inappropriate and indifferent (Malachi 1:10). Part of the purpose of His refining is to restore the worship of His people to something worth accepting (Malachi 1:11). While this may start with the priests, the people also have impure behaviors which need to be purged (Malachi 2:11–13).