John Calvin on Deuteronomy 32:15

15 But Jeshurun Lat., “Rectus.” See next note. 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 This word ישרון, yeshurun occurs only here, and in chap. 33:5, 26, and Isaiah 44:2. Commentators appear to be by no means agreed as to its derivation or meaning, — variously rendering it, the upright; the beloved; the fortunate; the abounding; the seer of God, etc. Singularly enough, C. himself, in his Commentary on Isaiah, (E Soc. Edit. vol. 3., p. 359,) gives the following contradictory opinion: “This designation is also bestowed upon that nation by Moses in his song: for although some render it in that passage Upright, and in this passage also, the old rendering is more suitable, “My beloved is grown fat.” (Deuteronomy 32:15.) 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.