I'm wondering why Jesus told Peter "Get thee behind me, Satan". What do you think Peter's initial reaction was?
I think if you look at the context of the verses to follow, you get a good idea of why Jesus rebuked Peter.
Matthew 16:23-28 (KJV)
23 But he turned, and said unto Peter, Get thee behind me, Satan: thou art an offence unto me: for thou savourest not the things that be of God, but those that be of men. 24 Then said Jesus unto his disciples, If any man will come after me, let him deny himself, and take up his cross, and follow me. 25 For whosoever will save his life shall lose it: and whosoever will lose his life for my sake shall find it. 26 For what is a man profited, if he shall gain the whole world, and lose his own soul? or what shall a man give in exchange for his soul? 27 For the Son of man shall come in the glory of his Father with his angels; and then he shall reward every man according to his works. 28 Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
In verse 23, after Christ rebukes Peter, He says the Peter does not value the things of God but he does value the things of men. Simply put, Peter was thinking purely about the physical world and not the spiritual world. Jesus had told his disciples that His kingdom was not of this world. The disciples are very slow to catch on.
In verse 24, Jesus says that you need to “lose” your life to follow Christ. This is a reference to not attempting to gain heaven by physical means, not to rely on yourself, ie trying to keep the OT covenant of the law by holy moral living. Jesus says that you need to “lose” your life or to be born again by the Spirit of God through faith in Christ via the NT covenant of grace. The rest of the verses support this dichotomy.
Verse 25 says that if you attempt to save your life (by your own attempt at righteousness) you will lose it (or taste spiritual death). However, if you lose your life (allow Jesus’ sacrifice on the cross to cover your sin by faith and trust in Him) then you will find your spiritual life and attain heaven.
Verses 26-27 is the result of trying to save your own life mentioned in verse 25. Jesus says what is the point if a man gains the whole world (through his physical good works) but he loses his soul (dies a spiritual death). He says if you are bent on living in the physical, then you will get the physical results and that is, according to verse 37, a physical judge to judge you by your works. The result of that reality will be spiritual death for no one can be saved via their work.
However, if you “lose your life” (give it over to Christ by faith) then you will see the Son of man coming in His kingdom. The truth of that verse was seen by 11 of the 12 Apostles seeing the death the burial and the resurrection of Christ, ie the coming of the NT kingdom of God.
What prompted Jesus to say “Get thee behind me, Satan” to Peter in Matthew 16:23 was the same experience that prompted him to respond to Satan’s temptation the way he did in the wilderness after he fasted for 40 day and 40 nights. Jesus was not addressing Peter as Satan in Matthew 16:23 but rather the spirit behind his speech. The request that Peter was making on behalf of Christ was against his purpose on earth. By the spirit of discernment, Jesus could see clearly that Satan was speaking through Peter. He could see that Satan had returned to him again at that moment with his antics. It can be recalled during his wilderness experience, the Bible says,
And when the devil had ended every temptation, he departed from him until an opportune time. (Luke 4:13 ESV).
At the moment Peter started to speak, Jesus could see that Satan was at it again. As ESV puts it, Satan had cashed in on that speech as the “opportune time”, to return to Christ with his temptations. Jesus’ response to Peter was as sharp as the way he had responded to the last temptation in the wilderness.
And Jesus answered him, “It is said, ‘You shall not put the Lord your God to the test.’” (Luke 4:12 ESV)
Jesus said to Peter “Get thee behind me, Satan”. That sounded a little bit coerce. What prompted such a forceful response? Satan was putting him to a test speaking through Peter. Jesus necessarily had to be firm about his reaction to that temptation, because it was the most effective way he could resist the devil (James 4:7).
What was Peter's initial reaction? Evidently Peter was going to be dumbfounded. This is partly because Jesus had taken the context of their conversation to a higher level, to which he (Peter) had no answer.
Peter's initial reaction was to immediately stop denying the truth.
"31 Because of this I say to you, all sin and evil speaking shall be forgiven to men, but the evil speaking of the Spirit shall not be forgiven to men." (YLT)
Peter spoke from a desire of the heart, that he did not want to see Yeshua die. But, he spoke without thinking, and his words contradicted the words of His Savior, and those of the Holy Spirit from all of the OT prophesies that the Messiah would be sacrificed as the Lamb slain from the foundation of the world. When Peter denied this, he essentially called both Yeshua and the Holy Spirit a liar. It was a step toward blasphemy, and was an act which the devil excels in... lies. Equating either the Messiah or the Holy Spirit with the actions of a lying devil is blasphemy.
Yeshua put a very abrupt stop to it. Peter's words were a denial of the truth. For that moment in time, he was in the very same act as the devil, which denies the truth. Therefore, Yeshua called Peter "satan" to wake him up, and stop him from further blasphemy.