Last week Jesus called Peter Petros, the Rock, and made him the foundation stone of the Church. This week he calls him a rock again, but a skandalon, because in fleeing the cross he stands in the way of Christ's saving mission.
In Mt 16:22, Peter felt scandalized by Jesus. Later in his life, writing 1 Peter 2:8, Peter quoted Isaiah?s prophecy (Is 8:14-15) that the Christ will be a petra, a rock, to cause Isael to stumble. Jesus always scandalizes those waiting for a Messiah who will lead them to military victory over Israel's enemies. This scandal has everything to do with the cross. As St. Paul puts it, in 1 Cor 1:23, "we proclaim Christ crucified, a stumbling block to Jews and foolishness to Gentiles."
And so it is for us: when we proclaim the Apostolic Faith in Jesus as the Christ, God can build on the foundation He laid in Peter, but when we deny by our lips, or our lives, that salvation comes through the cross, then we are scandalized by Jesus. We refuse to let Christ be crucified in us and we become a scandal to the Body of Christ, causing people to fall.
This should help us better understand why Jesus calls Peter "Satan." Satan hates that God has made himself so vulnerable as to come in the flesh and take on, not only human weakness, but human sin and suffering. It may be human to flee our weaknesses and sufferings, only wanting God to build up our strengths, but in so doing we deny our need for the Savior. In rebuking Jesus, Peter is momentarily saying as Satan did, "if you are headed to the cross, I will not serve you!"
Ultimately, Peter did follow Christ to the cross. When that time came, he considered it too much of an honor to be crucified like His Lord; so, Peter was crucified upside-down. Likewise, let us embrace our human weaknesses, take up our crosses, and follow Him. That is, let us receive Reconciliation and the other sacraments, perform works of mercy, pray, fast, and bear hardship humbly. Sharing in His cross, we will share in His glory.
Photo credit: "Caravaggio-Crucifixion of Peter". Licensed under Public domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Caravaggio-Crucifixion_of_Peter.jpg#mediaviewer/File:Caravaggio-Crucifixion_of_Peter.jpg