Joshua the high priest was accused by Satan because his sins were unforgiven, symbolized by his filthy clothes. This is a picture of a believer who is under accusation because he has not been clothed by the righteousness of Christ. As high priest, he was symbolic of all of God's people who is under the blood covenant. As long as sin remains unforgiven, Satan will have a field day accusing God's people, making them feel guilty and condemned. But there is no condemnation to them who are in Christ. How can we be clothed with righteousness again?
Firstly, Joshua could not get rid of the filthy clothes by himself--someone has to do it for him. In this case, it was the Angel of the Lord who told those standing next to him to remove his filthy clothes and put on clean clothes, symbolizing his sins have been taken away. Most bible scholars acknowledge that this Angel of the Lord is the pre-incarnate Christ. So, whether it is pre-Cross Christ or post-Cross Christ, it is Jesus Christ who is the only One who can forgive and cleanse us from our sins.
Secondly, that Joshua as the high priest was accused by Satan is an evidence that even the most spiritual person can still sin. The Apostle John says to believers in 1 John 1:8-10, "If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness. If we claim we have not sinned, we make him out to be a liar and his word has no place in our lives." It has been taught that this verse is not for believers but for unbelievers. But if we understand English, we know that the pronoun "we" means "us"---viz., the Apostle John himself and all believers. Furthermore, that he mentions that "his word has no place in our lives" can only mean he is referring to believers, because an unbeliever by definition is someone who has not received God's word in his or her life.
Thirdly, we need to confess our sins to experience the power of Christ's forgiveness to resist Satan's accusations and be purified "from all unrighteousness." In other words, our filthy clothes stained by sins will be replaced with clean clothes of Christ's righteousness the moment we acknowledge we have sinned against God. I've witnessed repeatedly the power of confession among believers in countless Alpha weekends. To say that only non-believers need to confess their sins is to rob God's people of the peace that comes from experiencing the cleansing of their guilty conscience through the purifying spower of God's forgiveness: "since we have a great priest over the house of God, let us draw near to God with a sincere heart in full assurance of faith, having our hearts sprinkled to cleanse us from a guilty conscience and having our bodies washed with pure water" (Heb 10:21-22). Each time we sin, we experience a guilty conscience. So, we need to draw near to God to be washed clean through confession of our sins, just as the high priest who goes into the Holy of holies needs to confess his own sins and the sins of the people.
Like money transferred to our bank account, God's blessings in Christ remains dormant until we draw upon them one by one through an act of confession of faith, including forgiveness of sins. The armour we have to protect ourselves against Satan's accusations is the cleansing power of God's forgiveness obtained by confessing our sins and claiming Christ's righteousness.
Thank You Lord that forgiveness and cleansing is ours in Christ the moment we acknowledge our sins and claim the cleansing power of the Blood of Christ. Amen.