There are two interesting pieces of imagery used here in this verse. When Christ comes, He will perform two complementary works; He will purify some sinners and judge others. The images used for that purifying work, the refiner's fire and launderer's soap, stress both its thoroughness and its severity. Both purifying works are based on God's righteous standard, which is expressed in His law. The heat of the refiner's fire was intense, in order to separate the dross from the molten, pure metal. Similarly, the launderer washed clothes using strong lye soap, after which the clothes would be placed on rocks and beaten with sticks. If sinners prefer Christ's cleansing work to his judgment, this is the price that must be paid.
Critics of God's law lie by saying God would not have given us the perfect law that poor human beings cannot keep, and so Christ has corrected His government and sent His Son to put us under a relaxed authority. Jesus Christ has shown, on the contrary, how intimately the law must be internalized so as to be clear about what we must do to properly keep it in the letter and the spirit. There is never a moment when it ceases to exist. It is eternal in its governance and forever demanding obedience. Everywhere God's law reveals our serious deviations from the way of righteousness and shows us how far short we come of God's glory.
Christ, in addition to explaining the law and pointing out its spiritual character, also unveiled its living essence.
The image of a furnace, used earlier in Malachi 3:2, reappears in this portrayal. Earlier the image was used to describe a future work of purging or purifying, as a result of which God would cleanse His people and establish a generation of those who would live righteously and worship Him in truth.
It is the kind of discipline Jesus spoke of through His use of the image of the vine and its branches. He spoke of trimming the branches, which is a blessing for God's people, though painful. He speaks of this here in John 15, where he says:
However, the second use of the image of a furnace, in Malachi 4:1, is quite different. Here the object of the burning is not purification but rather the destruction of the immoral. Malachi 4:1 again.
There are two different messengers involved in that first verse. The first messenger Jesus said was John the Baptist. The second messenger was Jesus Christ, the messenger of the covenant, the one who came preaching the gospel of the Kingdom of God.
It is very obvious that Jesus did not fulfill verses two and three when He came His first time. He is therefore evidently talking about His second coming, when those things will occur. Now that becomes even more evident as you continue through the context in chapter three. As you get to the end of the chapter he is obviously talking about the second coming of Jesus Christ, chapter four is even more obvious.
Talking about a day of judgement:
It's obviously speaking there about His first coming, but there are intimations of the second.
That's obviously a reference to His second coming, and that is part of the message of these Two Witnesses.
