In today's gospel,Christ is talking about the necessity of his passion and death. And St Peter barges in and says “Heaven forbid. No way, Lord. This shall never happen to you!”
And of course what he betrays is his own fear of death, which reflects his fears about his own life! He knows the moments where life has been empty, he knows what it is to lose your direction, your identity - and so at the moment that Christ even breathes the words, Peter is there saying “No, this cannot happen”.
But Jesus shouts back, "Get thee behind me, Satan!" Heavy words. But for Jesus it is completely different, because the one quality which marked his short life out especially, was that he lived life to the full. He was alive to the leper, alive to the Scribes and Pharisees, alive to the crowds… and alive to the dying.
And when people met him that was what leapt out at them – a quality of aliveness born of an inner aliveness to God. When they say in St John's gospel, “in him was life”, it was no pious platitude, but a simple statement of the impression he made on people he met – “In him was life”.
Picture above: extract from the painting, ‘Get Thee Behind Me, Satan’ by James Tissot (1836–1902). This image was uploaded by the Brooklyn Museum as a content partnership, and is considered to have no known copyright restrictions by the institutions of the Brooklyn Museum.