Soon after Peter made His great confession that Jesus is the Christ, the Son of the living God, Jesus began to explain some things regarding His impending death. Jesus had been teaching about the kingdom for some time, but thus far He had revealed very little about His approaching suffering and resurrection (cf. John 2:19-22; 3:14; Matt. 12:38-40). However, since the apostles (via Peter) had just confessed Jesus as the Messiah, they needed to be taught regarding God's eternal plan for the Christ (i.e., for Him to suffer, die, and be resurrected). At this point they were still thinking in physical terms regarding the Messiah. Their conceptions about His ministry are crude and not compatible with the notion of Him suffering and dying.
Peter can't believe what Jesus said! So, he took Him aside and began to rebuke Jesus - "Far be it from You, Lord; this shall not happen to You!" Given Peter's response to Jesus' prophecy, he certainly understood Jesus' plain words regarding His suffering and dying. Yet, Peter seemed to fail to grasp the accompanying promise of a resurrection.
Peter's actions, though misguided, were unquestionably done with good intentions. He took Jesus aside to deliver his message privately. He evidently believed Jesus was discouraged and needed to be cheered up before His words disheartened the disciples. Peter, thinking he was doing good, was actually overstepping his authority in assuming he knew better than Jesus, the One he had recently confessed to be the Christ! It should be noted that although Peter clearly confessed Jesus to be the Christ, he did not have a clear understanding of the work of the Messiah.
"Get behind Me, Satan! You are an offense to Me, for you are not mindful of the things of God, but the things of men" - This public response from Jesus would've come as a shock to all present. Certainly he labeled Peter as "Satan" at that moment because he was acting as an "adversary," though unknowingly. As is seen in Matthew 26:39ff, Jesus did not want to suffer and die, and thus He needed to be encouraged to complete His mission, not be dissuaded from it. Peter was an offense to Jesus in the sense that he was tempting Him to not fulfill the very purpose for which He came to Earth (cf. Luke 19:10). Peter did not understand God's will in this matter but was being blindly guided by his own judgment and feelings.
It should be observed that although Jesus does rebuke Peter, He doesn't rebuke him in the manner in which He rebuked the devil in Matthew 4:10 ("Away with you, Satan!"). In telling Peter to get behind Him, He desired that Peter be His follower again and not a servant of Satan.
We will continue studying this narrative in our next lesson.