Why was the prophecy that the elder would serve the younger so shocking to Isaac and Rebekah?

The prophecy shocked Isaac and Rebekah because it reversed established Middle Eastern custom and inheritance law, under which the firstborn twin, even among twins, became the eldest son and rightful heir. It declared the younger, Jacob, would be ascendant and the one through whom the promises would come, contrary to human reason.

Les explains that the announcement was contrary to Middle Eastern custom. Even in the case of twins, whichever twin was born first would be the eldest son and the rightful heir of the inheritance. So the prophecy that "the elder shall serve the younger" flew in the face of what Isaac and Rebekah understood. It meant Jacob, the second twin, would be the ascendant one and the one through whom the promises would come. Les notes this is against human reason. He also says God, in His foreknowledge before those lads were ever born, being able to see the end from the beginning, knew that Esau would be destitute of faith. This fits the purpose of God according to election, not of works, but of Him that calleth.

Romans 9:12

It was said unto her, The elder shall serve the younger.

What did God tell Rebekah about Esau and Jacob?

God told Rebekah that the elder, Esau, would serve the younger, Jacob, and this was declared before the boys were born. Scripture also states, "Jacob have I loved, but Esau have I hated," a comparative statement of God's surpassing love for Jacob.