John Calvin Commentary Genesis 27:40

"And by thy sword shalt thou live, and thou shalt serve thy brother. And it shall come to pass, when thou shalt break loose, That thou shalt shake his yoke from off thy neck." — Genesis 27:40 (ASV)

By thy sword shalt thou live, and shalt serve thy brother. It should be observed that events are predicted here that were never fulfilled in the person of Esau; and therefore, the prophecy concerns things that were, at that time, far distant. For Jacob was so far from having obtained dominion over his brother that, on his return from Padan-aram, he humbly submitted to him; and the breaking of the yoke that Isaac mentions here refers to a very remote period.

He is therefore describing the future condition of Esau’s posterity. And he says first, that they will live by their sword. These words have a twofold meaning: either that, being surrounded by enemies, they will live a warlike and restless life; or that they will be free and their own masters.

For there is no power to use the sword where there is no liberty. The first meaning seems more suitable: namely, that God would limit His promise, lest Esau be too greatly exalted, for nothing is more desirable than peace. The holy people are also warned that there will always be some enemies to harass them.

This, however, is a very different thing from living by his own sword. It is as if Isaac had said that the sons of Esau, like robbers, would maintain their security by arms and violence rather than by legitimate authority. A second limitation of the promise is that, though armed with the sword, he would still not escape subjection to his brother.

For the Idumeans eventually became tributaries to the chosen people. However, this servitude did not last long because when the kingdoms of Israel and Judah were divided, the power by which the chosen people had held all their neighbors in subjection and fear was cut off. Yet, the Lord would have the Idumeans brought into subjection for a short time, so that He might provide a visible demonstration of this prophecy.

For the rest of the time, the restless and unbridled liberty of Esau’s descendants was more wretched than any state of subjection.