"After Isaac finished blessing him, and Jacob had scarcely left his father's presence, his brother Esau came in from hunting. He too prepared some tasty food and brought it to his father. Then he said to him, 'My father, please sit up and eat some of my game, so that you may give me your blessing.' His father Isaac asked him, 'Who are you?' 'I am your son,' he answered, 'your firstborn, Esau.' 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!'" NIV translation
Just after Jacob left, Esau came in with the food to get his father's blessing. Isaac found out that he had been tricked and was shaken. Isaac knew that, once the words of blessing were expressed, God would hear and they would be fulfilled. The same is true for us, as Christians. We have the same type of relation to God as Isaac did in that, if we express something in words, we can be sure that God hears and will act in our best interests.