And the LORD said unto her, Two nations are in thy womb, and two manner of people shall be separated from thy bowels; and the one people shall be stronger than the other people; and the elder shall serve the younger. (Gen. 25:23)
The contention between the twin-brothers had reference to something far beyond their own persons; for in this way he shows that there would be discord between their posterities. When he says, there are two nations, the expression is emphatical; for since they were brothers and twins, and therefore of one blood, the mother did not suppose that they would be so far disjoined as to become the heads of distinct nations; yet God declares that dissension should take place between those who were by nature joined together.
Secondly, he describes their different conditions, namely, that victory would belong to one of these nations, forasmuch as this was the cause of the contest, that they could not be equal, but one was chosen and the other rejected. For since the reprobate give way reluctantly, it follows of necessity that the children of God have to undergo many troubles and contests on account of their adoption.
Thirdly, the Lord affirms that the order of nature being inverted, the younger, who was inferior, should be the victor. We must now see what this victory implies. They who restrict it to earthly riches and wealth coldly trifle. Undoubtedly by this oracle Isaac and Rebekah were taught that the covenant of salvation would not be common to the two people, but would be reserved only for the posterity of Jacob. In the beginning, the promise was apparently general, as comprehending the whole seed: now, it is restricted to one part of the seed. (Commentaries)