Romans 9:10

And not only this - Not only is the principle of making a distinction among the natural descendants of Abraham thus settled by the promise, but it is still further seen and illustrated in the birth of the two sons of Isaac. He had shown that the principle of thus making a distinction among the posterity of Abraham was recognised in the original promise, thus proving that all the descendants of Abraham were not of course to be saved; and he now proceeds to show that the principle was recognised in the case of his posterity in the family of Isaac. And he shows that it is not according to any natural principles that the selection was made; that he not only made a distinction between Jacob and Esau, but that he did it according to his good pleasure, choosing the younger to be the object of his favor, and rejecting the older, who, according to the custom of the times, was supposed to be entitled to special honor and rights. And in order to prove that this was done according to his own pleasure, he shows that the distinction was made before they were born; before they had formed any character; and, consequently, in such a way that it could not be pretended that it was in consequence of any works which they had performed.

But when Rebecca - The wife of Isaac; see Genesis 25:21, Genesis 25:23.