5 “He has chosen an inheritance for us, the excellency of Jacob, whom He loved”. A certain beauty of Jacob He has chosen for our inheritance. Esau and Jacob were two brothers; in their mother's womb both struggled, and by this struggle their mother's bowels were shaken; and while they two were yet therein, the younger was elected and preferred to the elder, and it was said, “Two peoples are in your womb, and the elder shall serve the younger.” Among all nations is the elder, among all nations the younger; but the younger is in good Christians, elect, godly, faithful; the elder in the proud, unworthy, sinful, stubborn, defending rather than confessing their sins: as was also the very people of the Jews, “being ignorant of God's righteousness, and going about to establish their own righteousness.” But for that it is said, “The elder shall serve the younger;” it is manifest that under the godly are subdued the ungodly, under the humble are subdued the proud.
Esau was born first, and Jacob was born last; but he who was last born, was preferred to the first-born, who through gluttony lost his birthright. So you have it written, He longed for the pottage, and his brother said to him, If you will that I give it you, give me your birthright. He loved more that which carnally he desired, than that which spiritually by being born first he had earned: and he laid aside his birthright, that he might eat lentils. But lentils we find to be the food of the Egyptians, for there it abounds in Egypt.
Whence is so magnified the lentil of Alexandria, that it comes even to our country, as if here grew no lentil. Therefore by desiring Egyptian food he lost his birthright. So also the people of the Jews, of whom it is said, “in their hearts they turned back again into Egypt.” They desired in a manner the lentil, and lost their birthright.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)