Genesis 25:22
And the children struggled together within her; and she said, If it be so, why am I thus? And she went to enquire of the LORD. And the children struggled together within her; and she said, If it be so, why am I thus? And she went to enquire of the LORD.
The struggling of the children together within her troubled Rebekah, so she went to inquire of the Lord in troubled prayer to seek an explanation of what was happening to her. The Lord answered her prayer. In His answer He revealed that she would have two sons. He further revealed their future, that they would become two nations, that one nation would be greater than the other, and that the elder would serve the younger.
The verse stands at the beginning of the future history of Jacob and Esau. The answer rested on divine foreknowledge, for God knows the end from the beginning and already knew before their birth what characters Jacob and Esau would develop.
That foreknowledge included that Esau would not have a heart to obey Him. For the elder to serve the younger overturned the peculiar advantages of the first-born, namely honor and authority next to the parents, consecration to God for the priest's office, and a double portion of the father's goods.