Zechariah 3:15 is the Vision of the High Priest. Notice 3:1, “Then he showed me Joshua the high priest standing before the Angel of the LORD, and Satan standing at his right hand to oppose him.”
Here we find the purpose of Satan. He comes to accuse or oppose Joshua the high priest. The name Satan means adversary, or one who opposes. Satan comes to bring complaints against Joshua being the high priest. However, you need to read verse 2: “And the LORD said to Satan, ‘The LORD rebuke you, Satan! The LORD who has chosen Jerusalem rebuke you! Is this not a brand plucked from the fire?”
So, here we have the priest chosen by God being accused or opposed by Satan. This happens in churches all of the time. Someone gets upset at the preacher. That person then poisons the minds of others in the church against the preacher. Satan is using that person or persons to destroy the unity of the church and sow discord among the members. Before long that preacher is convinced that the church will be better off without him. He then resigns, moves, or retires.
I think this is what we see in these verses. However, the LORD stands with the preacher. The LORD refines the preacher. Joshua, the High Priest, stays and Satan is dealt with. The LORD cleans the high priest up. He shows His support of the preacher: “And the Angel of the LORD stood by.”
Being a preacher is a hard job because most of your critics talk about you behind your back. They work behind the scenes until they have won more people to their point of view. Usually when the preacher finds out what is happening, it is too late for him to do anything about it. No matter what he says or does, the church decides it is best for him to be gone, so things will operate smoothly without any complaints. God did not let this happen to Joshua. He stood by him and God helped him become what he needed to be.