Peter had just declared that Jesus was the Christ (Messiah), the Son of the living God (Matthew 16:16).  It was God's direct revelation to him, and Jesus declared that such faith would become the foundation of the church (Matthew 16:18-19).  Peter would have felt that he was certainly 'on-side' with Jesus. It was the 'high point' of the disciples' interaction with Jesus: they were now confident they knew who He was.  But, at the same time, the divine countdown to the cross had begun.
 
However, when Jesus started to unpack the huge changes which would take place shortly, the disciples were shocked.  The one sentence summary of the trip to Jerusalem and death deeply offended them.  The prospect of Christ submitting to violence took them by surprise and the promise of His resurrection was no comfort.  Their spokesman, Peter, decided that Jesus must be wrong and rebuked Him.  His agenda for his hero Master was totally different.  But it was Peter who was wrong, and Jesus told him in no uncertain words, "Get behind me Satan."
 
It was now Peter who had become a scandal to God's kingdom: he was standing in the way of God's salvation plan.  He was not on-side at all – instead he was an obstacle who had to be removed.  It was as if Satan himself was standing in front of Jesus inspiring the evil words which quickly poured out of Peter's mouth.  All that the disciple could see was his own hope of glory (standing next to Jesus when the crowd proclaimed Him as king) disappearing, and maybe he would suffer too.  Which is exactly what Jesus went on to explain (Matthew 16:24-27).
 
The rebuke was as strong as any person ever received from the Lord.  And yet how often are we tempted to be an obstacle to hinder God's plans … and we do not know it.  We are just thinking of our own concerns; while pursuing our own agenda we are oblivious to the grief we are giving to the Lord.  We may even fail to hear His rebuke.  It is time to repent of setting our self-centred thoughts as an obstacle to His purposes.  It is time to be truly 'on-side', thinking about what the Lord is doing and how He wants to use us in His way, not ours.