6 He however, to signify that He is far from coming to the passion against His will, both rebuked Peter, and called him Satan.
Let them hear, as many as are ashamed of the suffering of the cross of Christ. For if the chief apostle, even before he had learned all distinctly, was called Satan for feeling this, what excuse can they have, who after so abundant proof deny His economy? I say, when he who had been so blessed, who made such a confession, has such words addressed to him; consider what they will suffer, who after all this deny the mystery of the cross.
And He said not, “Satan spoke by you,” but, “Get behind me, Satan.” For indeed it was a desire of the adversary that Christ should not suffer. Therefore with such great severity did He rebuke him, as knowing that both he and the rest are especially afraid of this, and will not easily receive it.
Therefore He also reveals the thoughts of his mind, saying, “Thou savorest not the things that be of God, but those that be of men.”
But what means, “Thou savorest not the things that be of God, but those that be of men”? Peter examining the matter by human and earthly reasoning, accounted it disgraceful to Him and an unmeet thing. Touching him therefore sharply, He says, “My passion is not an unmeet thing, but you give this sentence with a carnal mind; whereas if you had hearkened to my sayings in a godly manner, disengaging yourself from your carnal understanding, you would know that this of all things most becomes me. For thou indeed supposest that to suffer is unworthy of me; but I say unto you, that for me not to suffer is of the devil's mind;” by the contrary statements repressing his alarm.
Thus as John, accounting it unworthy of Christ to be baptized by him, was persuaded of Christ to baptize Him, He saying, “Thus it becomes us,” and this same Peter too, forbidding Him to wash his feet, by the words, “You have no part with me, unless I wash your feet;” even so here too He restrained him by the mention of the opposite, and by the severity of the reproof repressed his fear of suffering.
Source: Homilies on the Gospel of St. Matthew (New Advent)