Matthew 16:22-23
Then Peter took Him, and began to rebuke Him, saying, Be it far from thee, LORD: this shall not be unto thee.

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.

Peter had a way of speaking before thinking, and that day was no different. Peter was thinking with his heart and not his head. He was emotional about his Friend going to the cross to die for things that He had not done. He was not rationalizing what Jesus had taught him and the other apostles about His obedience to the Father. This was God's plan, and Peter was not focused on that at the moment. He was thinking of the temporary, as humans often do.

The LORD quickly and sternly rebuked Peter. He likened him to satan, who does not care about the things of God...as a matter of fact, satan opposes the things of God! That is what Peter was doing at that moment. He was thinking in the flesh, not in the Spirit.

We often think we have a better plan. Many times we do not like what God has ordained....I know that is true for me! My personal opinion is that Peter had no malicious intent at that moment. That does not mean that he was not wrong; I just think that he did what humans do. He was operating out of the flesh. But God knows more than we know. God sees the big picture. God knows the future, while our feeble minds can barely consume the present moment. God had a plan then, and He still has a plan.