
In today's Gospel we have a very peculiar interaction between Jesus and Peter. In the passage preceding the one you heard today, Jesus declared that Peter is the rock on which he will build his Church.
Now in today’s passage he calls him a satan and accuses him of trying to make him trip and fall. What gives? Peter is OK with believing that Jesus is the Messiah, the Son of the Living God, but he can't handle the thought of Jesus' death. But this is the course he is on and so Peter's opposition to the cross is not helpful — indeed, his concern for Jesus' physical well being is an obstacle and a temptation.
Earlier the real Satan had tempted Jesus in the desert and at that time Jesus refused to use his miraculous powers to relieve his hunger, or to impress unbelievers or take a short-cut to establishing his kingdom. Now Peter tempts him as he begins to face the cross, and here Jesus steps over the trip hazard of Peter's misplaced concern: he will go to the cross.
“That’s what Jesus is telling us. The cross is the price of our salvation — Jesus’ cross, of course. But also, the crosses that we have to bear — and I can give you a long list of crosses.”