Jesus reacted strongly to Peter’s reaction, because it was painful and astonishing to him. Peter had taken him aside (by the hand?) to quietly rebuke and comfort him, but Jesus pulled himself out of that isolation, to say, while facing the other disciples, who stood nearby: "Get behind me, Satan!”.
We try to avoid suffering, even when we should accept it in the plan of the Lord. Since the fall into sin, sin is our default guide, as it is here for Peter. For Jesus, the temptation is like that in the wilderness, with Peter like Satan to Jesus, seeking to turn him away from his work and to lead him to glory without suffering.
Peter’s reaction was clearly painful to Jesus and he was astonished at what the apostle said. This is why he reacts so vehemently to it. Peter had taken him aside (by the hand?) to a quiet place to admonish and comfort him. Yet Jesus wrenches himself out of that isolation, turns to Peter and proclaims—while facing the other disciples, who stand nearby: Get behind me, Satan!
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One must remember that human beings generally avoid suffering, even when they ought to accept it for the sake of the Lord’s cause. Since the fall into sin, what is human has become sinful. It is with this merely human mindset that Peter is occupied. For Jesus, the temptation lies in the same line as that in the wilderness. Peter becomes like Satan to Jesus, seeking to turn him away from his work and to lead him to glory without suffering—the very glory of which the apostle had confessed.2
23 But he turned and said to Peter, “Get behind me, Satan! You are a hindranceto me. For you are not setting your mind on the things of God, but on the things of man.”