Mark 8:33

But he turning about, and seeing his disciples, rebuked Peter, and saith, Get thee behind me, Satan; for thou mindest not the things of God, but the things of men.

The words of Christ here do not mean that Peter was satanic and depraved, but that in opposing the cross he was unconsciously taking the part of Satan in opposing the divine will. He was looking at things in the light of mere human wisdom and not from the perspective of the will of God. As Erdman said:

The offense of the cross has never ceased. It is still human and natural to insist that the death of Christ was not necessary; but the preaching of the cross is the very wisdom and the power of God.[15]

It should be noted that Christ did not pause to justify the cross by citing the benefits that would be achieved through such a means, setting forth the principle that it was the will of God, that being the only justification needed.

Turning about ... The presence of the other apostles was noted by Christ, and it therefore became necessary to rebuke Peter before them all and in terms that could leave no misunderstanding of the truth.

Get thee behind me, Satan ... This is generally interpreted to mean that Peter had gone out in front of the Lord in an effort to guide him, and the Lord was here ordering Peter to assume his proper place behind the Lord as a devoted follower and disciple.