Deuteronomy 32:15

15But Jeshurun (264) waxed fat. Moses here severely censures the ingratitude of the people, because when filled with delicacies, they began to wax wanton against God; for, according to the vulgar proverb, satiety breeds violence; but this arises from men’s detestable depravity, who ought rather to be inclined to humility and gentleness by the loving-kindness of God, since the more abundantly He supplies us with food, the more does He invite us to show forth the affection that becomes children, inasmuch as He thus more closely and familiarly declares Himself to be our Father. Intolerable, then, is the impiety of profane persons, who increase in insolence against Him, when they have gorged themselves with an abundance of all good things. They are here compared to restive horses, which, if they are well fed, without exercise, kick under their rider, and are rendered almost intractable. By using the word “upright” for Israel, he ironically taunts them with having departed from rectitude, and, reminding them of the high dignity conferred upon them, more severely reproves their sin of unfaithfulness. For elsewhere (265) Israel is honored with the same title without any evil imputation in respect to their calling; but here Moses reproachfully shows them how far they had departed from the pursuit of that piety, to the cultivation of which they had been called.