See Joel 2:11. The Jews believed that the Messiah was coming to punish the heathen in judgment. On the contrary, Malachi warns the Jews they will be the first to suffer judgment (see Amos 5:18).
Refiner’s fire.
As a fire separates the metal from the dross, so God by His judgment separates the righteous from the wicked (see on v. 1).
Fullers’ soap.
Not a true soap, which was probably unknown in ancient times, but a vegetable alkali obtained from the burning of certain plants, and used for washing purposes.