There is an incredible turn-around in the gospel text for this Sunday (Matthew 16:21-28). In last week's text, Jesus said to Peter, 'Blessed are you....' This week, in the text which follows immediately, he says to Peter, 'Get behind me, Satan.'
To call someone blessed, and then to call someone 'Satan', now THAT is a significant change. It seems as if Jesus is employing 'hate speech', that he is attacking someone. I am pretty sure he made Peter feel quite badly.
I also think it was important that Jesus made sure that he was clearly understood. Part of his role as Son of Man ... Son of David ... Son of God is to be a judge. A judge has the job of being a judge, making judgments. Sometimes another vocation carries that responsibility of judge as well.
The writer of this account brings out that some religious leaders were saying that Jesus was basically Satan because he was able to cast out demons. Jesus makes a good point when he says that, if Satan casts out Satan, his kingdom will not last for long. And he adds, 'And if I cast out demons by Beelzebul (i.e., Satan), by whom do your sons cast them out? Therefore they will be your judges (Matt 12:27).'
Jesus lets other people be judges as well. He helps others to understand the situation more clearly. He helps others to understand why he is there, in the midst of all this trouble and turmoil.
I would like to make the point that Jesus does not say to Satan, 'Go away.' He said that to him at the beginning of the account (Matt 4:10). In the Gospel according to Luke, the clarifying point is made, that Satan leaves 'until an opportune time' (Luke 4:13b). In the Gospel according to Matthew, when Jesus is getting closer to the cross, he says to Satan, 'Get behind me.' Although the name Satan does not show up any more in this account, he will be present in many of the coming events.
Jesus is, quite clearly, the leader. Satan has to get behind; and he does not like that position at all. We, on the other hand, do not mind it.
Jesus, you see, is headed in a wonderful direction. It is for our benefit.