John Calvin Commentary Genesis 25:33

"And Jacob said, Swear to me first. And he sware unto him. And he sold his birthright unto Jacob." — Genesis 25:33 (ASV)

And Jacob said, Swear to me. Jacob did not act cruelly towards his brother, for he took nothing from him, but only desired a confirmation of that right which had been divinely granted to him; and he does this with a pious intention, so that he may in this way more fully establish the certainty of his own election.

Meanwhile, the infatuation of Esau is to be observed, who, in the name and presence of God, does not hesitate to sell his birthright. Although he had before rushed inconsiderately upon the food under the maddening impulse of hunger, now, at least, when an oath is exacted from him, some sense of religion should have come over him to correct his brutal greed. But he is so addicted to gluttony that he makes God himself a witness to his ingratitude.