Peter believed that Jesus was the Messiah. But what was wrong here? Peter also disagreed with how God would work out His purpose through Christ. What he objected to was his good Friend having to go through a scourging and a painful and shameful crucifixion, which is a terrible way to die, especially for One so good. To him, it was unthinkable that Jesus should suffer all the ignominy and be berated by those in authority—and Peter recognized that those in seats of authority could not hold a candle to Jesus. Yet these mean men would sit in judgment of Him and actually deliver Him to death.
Peter disagreed with what his Messiah said God's purpose was and how it would be worked out. We can relate to what Peter said. It really was a touching sentiment because he did not want to see Christ suffer and die, but the sentiment was wrong. Christ identified the source of what Peter said as Satan.
How did He isolate that and conclude it was from Satan? For one, it followed the same pattern as Satan's temptations in Matthew 4: offering Christ Messiahship without suffering. "Just bow down to me, and I'll give you all the kingdoms of the world. You don't have to suffer, Jesus." (This last bit Satan implied.)
Satan knew the Scriptures. He knew who Jesus was, and he also knew the Scriptures better than Peter did. Satan tossed in front of Christ the temptation of achieving Messiahship, rulership over the world, without having to go through the ignominy of scourging and death by crucifixion.
It was quite a temptation. Probably most of us would have taken the out. Jesus, though, recognized it right away.
It was not God's will. His will was that Christ first had to suffer and then die for man's sins. Where does it say that in God's Word? Isaiah 52 and 53 are clear about God's will for the Messiah.
When he spoke, Peter was not speaking God's words or thoughts regarding the Messiah. Instead, he was communicating what he would like to see occur. But God's thoughts are not man's thoughts. What Peter spoke suggested the common Jewish conception of a Warrior-Messiah who would put down Judah's enemies, elevating her over her conquerors, and she would become the kingpin of all the nations on earth. This would bypass the idea of a suffering Messiah, who dies for the sins of man. But God had willed first things first: The Christ had to suffer and die before He could become King of kings and put down His foes.
Where did Peter get the idea of a Warrior-Messiah? Peter was a victim of a demonic disinformation campaign regarding God's Word, and by believing it and acting on it, he became a stumbling-block to others. The disinformation came from Satan through his false prophets.