Jesus is not suggesting that Peter has been possessed by Satan. Rather, Jesus is saying that Peter’s attempts to stop him from fulfilling his mission is in keeping with what Satan wants. Satan wants to stop Jesus from doing his work of redemption and now Peter tries to do exactly the same.
33 But turning and seeing his disciples, he rebuked Peter and said, “Get behind me, Satan! For you are not setting your mind on the things of God, but on the things of man.”