MATTHEW 16 21 From that time Jesus began to show to His disciples that He must go to Jerusalem, and suffer many things from the elders and chief priests and scribes, and be killed, and be raised the third day. 22 Then Peter took Him aside and began to rebuke Him, saying, “Far be it from You, Lord; this shall not happen to You!”23 But He turned and said to Peter, “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.”24 Then Jesus said to His disciples, “If anyone desires to come after Me, let him deny himself, and take up his cross, and follow Me. 25 For whoever desires to save his life will lose it, but whoever loses his life for My sake will find it. 26 For what profit is it to a man if he gains the whole world, and loses his own soul? Or what will a man give in exchange for his soul? 27 For the Son of Man will come in the glory of His Father with His angels, and then He will reward each according to his works. 28 “Assuredly, I say to you, there are some standing here who shall not taste death till they see the Son of Man coming in His kingdom.”

The very one who had obtained a revelation, who had been blessed, has now so soon fallen away, so as now to fear the Passion of the Lord, and thereby his faith has been overthrown. It is remarkable that Peter, who had not yet been fully instructed in the course of revelation, should come up with these responses. The larger picture had not yet been revealed to Peter, and he was confused and overwhelmed. Peter had learned that Christ is the Son of God. But he had not learned of the mystery of the cross and the resurrection. It was as yet not manifested to him. It remained hidden. Do you see how correct Jesus was in forbidding them not to declare his identity publicly? For if it so confounded the disciples, who were being made aware of it, who knows what the response of others might have been. This is why he rebuked Peter and called him Satan: to signify that he is coming to his future suffering voluntarily.
(St John Chrysostom)