In Matthew 16:18 Jesus calls Peter the rock, on which He would build His church and even says that He would give him the key to the Kingdom of Heaven.... But only a few verses on Jesus says to Peter: „Get behind me, Satan!” What am I to believe now? Is this Peter Satan or is he the keeper of the keys to the Kingdom of Heaven?
Answer: Matthew 16:18 reads: And I tell you that you are Peter [meaning rock], and on this rock I will build my community, and the gates of Hades will not overcome it. This means that the powers of evil, which draw mankind first of all towards the death of sin and then later into eternal death, will not be able to get through the gates of the underworld. It will be the task of the Church to rescue the chosen ones from the reign of death, from actual and especially from eternal death, to enable them to enter the Kingdom of Heaven. (see Colossians 1:13, 1 Corinthians 15:26, Revelation 6:8, 20:13). Like the city of death, the city of God too has gates. They permit entry only for those who are worthy (see Matthew 23:13). Peter receives the keys to those gates. It will therefore be his task, mediated through the Church, to open or shut the gates to the Kingdom of God.
Matthew 16:23 reads: „Get behind me, Satan! You are a stumbling-block to me; you do not have in mind the things of God, but the things of men. In Matthew 16:21 Jesus had announced His coming suffering for the first time and declared He would have to go to Jerusalem and suffer many things. That, yes, He would be killed, but that He would be raised to life on the third day. Thus Jesus combines the glorious task of the Messiah (see Habakkuk 3:13, Samuel 2:10, Psalms 2) with the painful task of the suffering servant of God (see Isaiah 42:1-9,. 49:1-6, 50:13-52, 12). Jesus prepares the disciples faith for the coming crisis of His death and resurrection. You are a stumbling-block to me, meaning, you are my potential fall (death). Because Peter wants Jesus to resist suffering and His shameful death, as he does not understand the road the Messiah must take, he becomes His stumbling block (the original meaning of the Greek word skándalon) und the, albeit unwitting, helper of Satan. In Matthew 4:1-10, the gospel writer had already told of Satan’s attempt to tempt Jesus in the wilderness in order to divert Him from His path of suffering and the humility of being Gods servant. Peter and the other disciples hoped right until Jesus death that He would eventually use His powers and proclaim victory over His enemies on earth. Only when the Holy Spirit came upon them did Peter and the other disciples receive the grace of understanding the true meaning of the cross and the resurrection. All those who follow Jesus as His disciples will have to walk the same rocky and stony path, will have to learn that the path of the Messiah is a path of service and suffering. This is especially true for the successors of Peter in the pastoral ministry of the Church.