Genesis 27:33

trembled very exceedingly Isaac's agitation is expressed in the original with an emphasis which our version can hardly reproduce. Lat. "expavit Isaac stupore vehementi et ultra quam credi potest admirans," where two renderings seem to be combined. LXX ἐξέστη ἔκστασιν μεγάλην σφόδρα.

and he shall be blessed Isaac, even in this moment of agitation, admits that the blessing of the firstborn has been pronounced with all due regularity, and is irrevocable. The Divine purpose has been affirmed. Esau, the firstborn, has not received the blessing. It could, therefore, only have been Jacob, the supplanter, whose voice at the first Isaac had thought he recognized.