Here is a quote from the book:
"By this word fear, we are to understand God himself, who is the object of our fear: For the Divine majesty goeth often under this very name himself. This name Jacob called him by, when he and Laban chid together on Mount Gilead, after that Jacob had made his escape to his father's house; "Except," said he, "the God of my father, the God of Abraham, and the fear of Isaac had been with me, surely thou hadst sent me away now empty." So again, a little after, when Jacob and Laban agree to make a covenant of peace each with other, though Laban, after the jumbling way of the heathen by his oath, puts the true God and the false together, yet "Jacob sware by the fear of his father Isaac" (Gen 31:42,53)."
http://truthinheart.com/EarlyOberlinCD/CD/Bunyan/text/Treatise.Fear.God/Entire.Book.html