John Calvin Commentary Genesis 27:39

"And Isaac his father answered and said unto him, Behold, of the fatness of the earth shall be thy dwelling, And of the dew of heaven from above." — Genesis 27:39 (ASV)

Behold, thy dwelling shall be the fatness of the earth. Eventually, Esau obtains what he had asked. For, perceiving himself to be cast down from the rank and honor of primogeniture, he chooses to have prosperity in the world, separated from the holy people, rather than to submit to the yoke of his younger brother.

But it may be thought that Isaac contradicts himself in offering a new benediction, when he had previously declared that he had given to his son Jacob all that was placed at his disposal. I answer that what was previously said concerning Ishmael must be noted here.

For God, though he listened to Abraham’s prayer for Ishmael as far as it concerned the present life, yet immediately restricts his promise by adding the exception implied in the declaration that in Isaac only should the seed be called. I do not, however, doubt that the holy man, when he perceived that his younger son Jacob was the divinely ordained heir of a happy life, would endeavor to retain his firstborn, Esau, in the bond of brotherhood, so that he might not depart from the holy and elect flock of the Church.

But now, when he sees him obstinately tending in another direction, he declares what will be his future condition. Meanwhile, the spiritual blessing remains in its integrity with Jacob alone, to whom Esau, refusing to attach himself, voluntarily becomes an exile from the kingdom of God. The prophecy uttered by Malachi (Malachi 1:3) may seem to contradict this statement.

For, comparing the two brothers, Esau and Jacob, with each other, Malachi teaches that Esau was hated, since a possession was given to him in the deserts; and yet Isaac promises him a fertile land.

There is a twofold solution: either that the Prophet, speaking comparatively, could truthfully call Idumea a desert in comparison with the land of Canaan, which was far more fruitful; or that he was referring to his own times.

For although the devastations of both lands had been terrible, yet the land of Canaan in a short time flourished again, while the territory of Edom was condemned to perpetual sterility and given up to dragons.

Therefore, although God, with respect to his own people, banished Esau to desert mountains, he yet gave him a land sufficiently fertile in itself to make the promise by no means futile. For that mountainous region both had its own natural fruitfulness and was so watered by the dew of heaven that it would yield sustenance to its inhabitants.