14 “Turned be mine enemies backward”. This thing to these very men is profitable, no ill to these men he is wishing. For to go before they are willing, therefore to be amended they are not willing. Thou warnest your enemy to live well, that he amend himself: he scorns, he rejects your word: “Behold him that advises me; behold him from whom I am to hear the commandments whereby I shall live!” To go before you he wills, and in going before is not amended. He minds not that your words are not yours, he minds not that your life to God you tell out, not to yourself.
In going before therefore he is not amended: it is a good thing for him that he be turned backward, and follow him whom to go before he willed. The Lord to His disciples was speaking of His Passion that was to be. Peter shuddered, and says, “Far be it, O Lord;” he that a little before had said, “You are the Christ, Son of the living God,” having confessed God, feared for Him to die, as if but a man. But the Lord who so came that He might suffer (for we could not otherwise be saved unless with His blood we were redeemed), a little before had praised the confession of Peter....But immediately when the Lord begins to speak of His Passion, he feared lest He should perish by death, whereas we ourselves should perish unless He died; and he says, “Far be it, O Lord, this thing shall not be done.”
And the Lord, to him to whom a little before He had said, “Blessed you are, and upon this Rock I will build my Church,” says, “Go back behind, Satan, an offense you are to Me.” Why therefore “Satan” is he, that a little before was “blessed,” and a “Rock”? “For you savour not the things which are of God,” He says, “but those things which are of man.” A little before he savoured the things which are of God: because “not flesh and blood has revealed to you, but My Father which is in the Heavens.”
When in God he was praising his discourse, not Satan but Peter, from petra: but when of himself and out of human infirmity, carnal love of man, which would be for an impediment to his own salvation, and that of the rest, Satan he is called. Why? Because to go before the Lord he willed, and earthly counsel to give to the heavenly Leader. “Far be it, O Lord, this thing shall not be done.” You say, “Far be it,” and you say, “O Lord:” surely if Lord He is, in power He does: if Master He is, He knows what He does, He knows what He teaches.
But you will to lead your Leader, teach your Master, command your Lord, choose for God: much you go before, go back behind. Did not this too profit these enemies? “Turned be Mine enemies backward;” but let them not remain backward. For this reason let them be turned backward, lest they go before; but so that they follow, not so that they remain.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)