This vision that God gives Zechariah is a beautiful picture of God's cleansing work in the lives of his people. The High Priest, who represents his people, is standing in front of God in filthy clothes and Satan is hurling accusations at him. It is like he has been caught in the midst of some big sin and his clothes show everyone what he has done and Satan is going to make absolutely sure that everyone hears about it. This is the way Satan wants us to see ourselves so that it will keep us scared and ashamed. But then God speaks and rebukes Satan ... tells him to "shut up." He says, "The LORD who has chosen Jerusalem rebuke you! Is not this a brand plucked from the fire?” (v. 2) Hear what God is saying to Satan: "If I had wanted to burn him up then I would have let him burn, but I plucked him out of the fire and I am going to forgive and cleanse him." He then proceeds to put new clothes on Joshua.
This was a promise to God people then that God wasn't through with them and that He was ultimately going to send his Son into the world to cleanse them all "in a day". (vv. 8-9) We can look back on that promise as having been fulfilled in Jesus and realize that God is still doing that cleansing work in our lives today. Child of God, rejoice in the fact that if God had wanted to burn you up then He would have done so, but you have been plucked out of the fire and clothed with fresh garments. Thank God for giving us what we never could have earned.