In Matthew 16:19, Simon has been told that he is "Peter", the rock on which the Church is to be built. Now, just five verses later, Jesus calls him "Satan". Bad enough if Peter had dropped back down to being Simon again, but he seems to sink even further, down to the the devil.
What happened? What'd he do? For that, let's go back to the end of last week's Gospel.
Verse 20 tells us that Jesus "strictly ordered his disciples to tell no one that he was the Christ." He's let the cat out of the bag. He confirmed what they may have suspected, but now they got confirmation. They must've thought they had won "life's lottery": their longed for Messiah had arrived, and they were not just on the earth when it happened; they were part of his "inner circle"! Woo-hoo!
Now, fast forward to today's Gospel. While the Apostles are basking in their imagination of what it would mean to be a member of the Messiah's court, Jesus brings them back to earth with the reality of what his Kingship will mean for him: to "suffer greatly" and "be killed". His "red carpet" is the Way of the Cross. No "velvet ropes"; just the cords of a scourge. For Peter and the other eleven, this was like being told they were getting a pony for Christmas, only to find a pile of pony-poo under the tree.
Matthew tells us Peter "took Jesus aside". This is one of those little moments that really make no difference to the story itself, but gives us something that an eye-witness to the event (Matthew) would have remembered. The other eleven guys saw Jesus & Peter talking quietly, and then hear Jesus rebuke Peter loudly. They must've thought, "What did he say to Jesus?"
We know what Peter said. "No such thing shall ever happen to you." In other words, "I'll take care of it, Jesus. You don't have to suffer. You won't be killed." Jesus has heard that before, a while back in the desert just after he was baptized by John. Only then, it was the devil telling him something like that. He recognized that this wasn't Peter talking to him; it was the devil talking through Peter to him. Thus, the rebuke is not so much to Peter as, through Peter, back to Satan.
Perhaps it'd be a little easier for us to fight sin if we saw it coming a mile away. But more often than not, sin comes disguised, packaged nicely. Sometimes, the temptation to sin even comes from another person talking us into it. Sometimes that "other person" is a friend, even our best friend. The Devil is the master of knowing just what buttons to push in us. He knows our weaknesses, whether chronic or momentary, and he waits for just the right time to move in for the attack.
How do we fight an opponent like that? Like Jesus, we need to be able to identify Satan, even when he comes at us in disguise. Nothing is better for building our confidence than getting a good "first punch" in, and nothing accomplishes this better than announcing to Satan that you can see through his disguise.
Sometimes we're going to lose those fights, but sometimes we're going to win them. Cheer up, Peter. You had a bad round, but it's a long fight. In the wise words of Francis Albert, "Each time I find myself flat on my face, I pick myself up and get back in the race."