Isaac emulated what his father had done in Egypt and Gerar. In cowardice and unbelief he claimed that his wife was actually his sister. He was not young and dumb - no, he was a full-fledged independent adult; a successful man of experience and status... yet he acted like little wuss.
The old saying about the apple not falling far from the tree applies here. But bad behavior wasn't the only thing that the son continued. He also accepted the same promises from God (including the gospel which was again just a footnote within the apparition: "In thy seed [Messiah] shall all the nations of the earth be blessed”).
Notice also a surprisingly positive description of Abraham in Genesis 26:5. Not only does that verse present a doctrinal dilemma, it is a practical surprise. We have spent the last 2 weeks reading about Abraham's blunders; realizing the stark reality of his basic sinfulness. Yet God not only gave Abraham a glowing A+ grade on his report card, he told Isaac that his blessings were a result of his dad's obedience. Huh? Did God read the same account that we've been reading?
Here is the answer: Hebrews 1... all of Hebrews 11; but most specifically Hebrews 11:8-19. By faith, by faith, in faith, by faith Abraham embraced the promises of God, lived for the next life and believed that "GOD IS ABLE" . . . Whatever Abraham's many flaws (by our fallible judgment), God was "not ashamed to be called the God of Abraham" - why? Because, Abraham believed God and thereby passed the critical tests that God placed in front of him.
Well, despite Isaac's character flaws, he was God's chosen one and was therefore blessed (Genesis 25:13). His wealth was multiplied exponentially in a very short period of time until the Philistines began to envy and fear him because he was "mightier" than they. There was even some frustrating conflict between Isaac and his Philistine neighbors; conflict that manifested itself in hostility over several wells of water which Isaac's men dug (or re-dug). He named those wells Esek (contention), Sitnah (strife), Rehoboth (wide place) and finally Shebah (an oath). At Shebah Isaac finally made peace with the Philistines... and with God. Here again the continuity of God's doctrine is seen in Isaac's faith as he "called upon the name of the Lord" (Genesis 26:25 & Romans 10:13).