Genesis 27:33-37 New International Version (NIV)
33 Isaac trembled violently and said, “Who was it, then, that hunted game and brought it to me? I ate it just before you came and I blessed him—and indeed he will be blessed!”
34 When Esau heard his father’s words, he burst out with a loud and bitter cry and said to his father, “Bless me—me too, my father!”
35 But he said, “Your brother came deceitfully and took your blessing.”
36 Esau said, “Isn’t he rightly named Jacob? This is the second time he has taken advantage of me: He took my birthright, and now he’s taken my blessing!” Then he asked, “Haven’t you reserved any blessing for me?”
37 Isaac answered Esau, “I have made him lord over you and have made all his relatives his servants, and I have sustained him with grain and new wine. So what can I possibly do for you, my son?”
Before the father of a family would die, he would perform a ceremony of blessing where he would officially hand over the birthright to the rightful heir. The first born son was usually entitled to the birthright. This birthright was not his until the blessing was pronounced out loud by the father of the family. Now, before this blessing was given, the father had the right to take the birthright away from the oldest son and give it to a son he felt to be more deserving. Once the blessing was given and pronounced out loud, it could not ever be taken away. It was set in stone. This is why fathers usually waited to do this until late in life. Jacob had been given the birthright by his older brother years before, but this did not hold to be set until his father made the blessing to Jacob. After Isaac blessed Jacob, it was binding and could never be revoked.