G-d promises that his covenant to Avraham will be fulfilled through Yitzchak.
Now we know of course that Yitzchak, like Avraham before him and Yaakov after him, was totally righteous (albeit not totally perfect as nobody was), and worthy of being a Patriarch.
However Yaakov's choice wasn't automatic, in spite of the prophecy. How did G-d know that Yitzchak wouldn't choose to be sinful like his son Eisav?