Q. If Jesus was making Peter His Prime Minister in the Church then why did He call Peter Satan a few verses later?
A. Peter was still very human, subject to sin, and the influence of Our Enemy. Because, Peter, in trying to dissuade Jesus from the cross, came under the influence of Satan and tempted Jesus not to take up His cross, Jesus clearly perceived the true source of Peter’s seemingly compassionate words and denounces Satan and his influence on Peter.
“Get behind me Satan”. He was addressing Satan not calling Peter Satan. Then, speaking to Peter, Jesus says,
You are a stumbling block to me; you do not have in mind the things of God, but the things of men.”
This was Peter’s first lesson in learning how to discern the voice of Satan and the Voice of Christ.