One of Satan’s favourite ploys is to accuse us before God, pointing out that we have no righteousness of our own and therefore no right to stand in God’s presence. Once again, we are faced with a half truth that can easily lead us astray, if we are not careful. As a statement of fact, Satan is right to say that we are unworthy to stand before God, but in saying this he is not reckoning with God’s grace and mercy, to both of which he is a stranger.
A classic example that illustrates this occurs in Zechariah 3:1-2, where the prophet has a vision of the high priest who is clothed in filthy rags and is therefore unworthy to perform the all-important task of making atonement for the sins of the people. But the high priest, who is called Joshua, is saved by God, and Joshua’s atoning sacrifice is accepted because he has taken the sins of the people on himself. His filthy clothing is not a sign of his character but the sins of the people for whom he is making atonement, and the fact that the high priest takes this on himself reveals a deeper righteousness than anything that Satan can grasp.
The vision of Zechariah was fulfilled in the sacrifice of Jesus, when He became sin for us in order to take it away and make it possible for us to share in the righteousness of God Himself. We who are filthy inside and totally unworthy of God’s grace have been covered by a cloak of righteousness dyed in the blood of the One who was slain in our place. It is because of that covering that we who have no merit of our own have been made acceptable to God. Satan has no right to accuse those whom Jesus has chosen and united to Himself - because we have been set free from the condemnation that we would otherwise deserve. That does not stop Satan from trying, of course, and it is here perhaps more than anywhere else that we must be constantly on our guard, so as not to fall into the trap that he wants to set for us.
Gerald Bray, God is Love, pg. 363-364