Mark 8:31-33 “And he began to teach them, that the Son of man must suffer many things, and be rejected of the elders, and of the chief priests, and scribes, and be killed, and after three days rise again. And he spake that saying openly. And Peter took him, and began to rebuke him. But when he had turned about and looked on his disciples, he rebuked Peter, saying, Get thee behind me, Satan: for thou savourest not the things that be of God, but the things that be of men.”
When Jesus called someone a name, it was not because He was mean. He was not mean in any way. When He called Peter “Satan,” it was because Peter’s suggestion on this occasion came from the Devil. To fulfill Old Testament prophecy, Jesus would (1) have to go to Jerusalem, (2) have to be rejected by the leaders of Israel (the elders, chief priests and scribes), (3) have to be killed, and (4) would have to rise again from the dead. This was God’s will, God’s plan. It had to be done in order to save us from our sins. When Peter opposed this plan of God, he was motivated by the Devil to do so. Though well meaning, Peter was far from infallible. Those who try to rebuke God (contradict Him) will be rebuked by God. It may be in this life or in the world to come, but God’s rebuke will certainly happen.