3 This same Peter therefore who had been by the Rock pronounced “blessed,” bearing the figure of the Church, holding the chief place in the Apostleship, a very little while after that he had heard that he was “blessed,” a very little while after that he had heard that he was “Peter,” a very little while after that he had heard that he was to be “built upon the Rock,” displeased the Lord when He had heard of His future Passion, for He had foretold His disciples that it was soon to be.
He feared lest he should by death, lose Him whom he had confessed as the fountain of life. He was troubled, and said, “Be it far from You, Lord: this shall not be to You.” Spare Yourself, O God, I am not willing that You should die. Peter said to Christ, I am not willing that You should die; but Christ far better said, I am willing to die for you. And then He immediately rebuked him, whom He had a little before commended; and calls him Satan, whom he had pronounced “blessed.” “Get behind Me, Satan,” he says, “you are an offense unto Me: for you savour not the things that be of God, but those that be of men.” What would He have us do in our present state, who thus finds fault because we are men?
Would you know what He would have us do? Give ear to the Psalm; “I have said, You are gods, and you are all the children of the Most High.” But by savouring the things of men; “you shall die like men.” The very same Peter a little while before blessed, afterwards Satan, in one moment, within a few words! You wonder at the difference of the names, mark the difference of the reasons of them. Why do you wonder that he who was a little before blessed, is afterwards Satan? Mark the reason wherefore he is blessed.
“Because flesh and blood has not revealed it unto you, but My Father which is in heaven.” Therefore blessed, because flesh and blood has not revealed it unto you. For if flesh and blood revealed this to you, it were of your own; but because flesh and blood has not revealed it unto you, but My Father which is in heaven, it is of Mine, not of your own. Why of Mine? “Because all things that the Father has are Mine.” So then you have heard the cause, why he is “blessed,” and why he is “Peter.” But why was he that which we shudder at, and are loth to repeat, why, but because it was of your own? “For you savour not the things which be of God, but those that be of men.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)