One of the biggest challenges to overcome in our service for the Lord is our own guilt and feelings of unworthiness. Satan uses our failures to accuse us and resist us (“the accuser of our brethren” Rev 12:10). That is what we see happening in Zechariah 3.
Joshua, the high priest, who was the high priest when they came back into the land, is standing before “the angel of the Lord” who is also “the Lord” in v.2. That means, he is standing before God the Son. And Satan is there resisting him, very likely protesting Joshua and the people’s right to be used by God. His reason was their unworthiness, pictured in the filthy garments Joshua is wearing (v.3).
The Lord makes him worthy, by removing his iniquity from him and clothing him with new garments. A fair mitre (headband) is put on him, and then he is instructed to walk in the Lord’s ways and to serve the Lord.
This speaks of God’s forgiving of the people who returned to the land and of God’s future forgiveness when He returns to the earth (v.8-10). It also teaches us that our worth is not something we earn but is graciously given to us by God. He cleanses us and makes us worthy to serve. He answers our accuser with His blood. We can stand and serve God because of the work of Christ for us!