12 “By this I know that You favour Me, that Mine enemies shall not triumph over Me” Because the Jews did triumph, when they saw Christ crucified; they thought that they had fulfilled their will to do Him hurt: the fruits of their cruelty they saw in effect, Christ hanging on the Cross: they shook their heads, saying, “If Thou be the Son of God, come down from the Cross.” He came not down, who could; His Potency He showed not, but patience taught. For if, on their saying these things, He had come down from the Cross, He would have seemed as it were to yield to them insulting, and not being able to endure reproach, would have been believed conquered: more firm remained He upon the Cross, than they insulting; fixed was He, they wavering. For therefore shook they their heads, because to the true Head they adhered not. He taught us plainly patience. For mightier is that which He did, who would not do what the Jews challenged. For much mightier is it to rise from the sepulchre, than to come down from the Cross. “That Mine enemies shall not triumph over Me.” They triumphed then at that time. Christ rose again, Christ was glorified. Now see they in His Name the human race converted: now let them insult, now shake the head: rather now let them fix the head, or if they shake the head, in wonder and admiration let them shake....
13. “But as for Me, You uphold Me, because of my innocence”. Truly innocence; integrity without sin, requiting without debt, scourging without desert. “You uphold Me because of Mine innocence, and hast made Me strong in Your sight for ever.” You have made Me strong for ever, You made Me weak for a time: You have made Me strong in Your sight, You made Me weak in sight of men. What then? Praise to Him, glory to Him. “Blessed be the Lord God of Israel.” For He is the God of Israel, our God, the God of Jacob, the God of the younger son, the God of the younger people. Let none say, Of the Jews said He this, I am not Israel; rather the Jews are not Israel. For the elder son, he is the elder people reprobated; the younger, the people beloved. “The elder shall serve the younger:” now is it fulfilled: now, brethren, the Jews serve us, they are as our satchellers, we studying, they carry our books. Hear wherein the Jews serve us, and not without reason....With them are the Law and the Prophets, in which Law, and in which Prophets, Christ is preached. When we have to do with Pagans, and show this coming to pass in the Church of Christ, which before was predicted of the Name of Christ, of the Head and Body of Christ, lest they think that we have forged these predictions, and from things which have happened, as though they were future, had made them up, we bring forth the books of the Jews. The Jews forsooth are our enemies, from an enemy's books convince we the adversary....If any enemy clamour and say, “You for yourselves have forged prophecies;” be the books of the Jews brought forth, because the elder shall serve the younger. Therein let them read those predictions, which now we see fulfilled; and let us all say, “Blessed be the Lord God of Israel, from everlasting to everlasting, and all the people shall say, So be it, So be it.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)