Commentaries:
If we put this purging into a different metaphor, it means getting rid of the leaven (see I Corinthians 5:7); here people are being purged of sin. God shows that He will use persecution to purify His people through the testing of their loyalty. We know that our loyalty is being really tested all the time, but a time is coming when it will require a great deal more intense conviction about what we believe. This becomes very important because His purpose is to entrench these beliefs into our character.
If people are truly convicted regarding their beliefs, they will conduct themselves far differently than if they are unsure. It is right here that this point becomes important—super-important really—because the dross of cowardly believers is going to be removed. That is what Malachi 3:3 is about. It will either be removed or else. That is very sobering.
John W. Ritenbaugh
Does Doctrine Really Matter? (Part Four)