Rebekah and Jacob had enough faith, enough belief, that they lied to Isaac to get his blessing. They had faith, but not yet the morality that would come later with the Ten Commandments.
It is like, when I was two years old, I would lie to get a cookie. Because, when I was two years old, my parents had not yet taught me the importance of being honest. During the time when Rebekah and Jacob lived, we human beings as a whole did not yet know the importance of being honest.
But God does not wait for perfect people. God moves His plans forward working with real, flawed people, people like you and me. For us to help make God’s hopes and dreams come true, we do not have to be perfect, we just have to be willing to work with God.
Like a wise teacher he takes hold of us where we are and leads us progressively toward [Himself]. # 2607
Jacob did not know enough to value honesty, but he did know enough to value a blessing that came from God through Isaac.
Yet Jacob was so typically human.
Jacob … made this vow: “If God remains with me, to protect me on this journey I am making and to give me enough bread to eat and clothing to wear, and I come back safe to my father’s house, [THEN] the LORD shall be my God. Gen 28:20-21
Well, God did remain with Jacob and Jacob was successful through many adventures.
So Jacob told his family and all the others who were with him: “Get rid of the foreign gods that you have among you; (their little carved idols, their false gods) … I will build an altar … to the God who answered me in my hour of distress and who has been with me wherever I have gone.” Gen 35:2-3
Later on, God would tell Moses,
… the God of Abraham, the God of Isaac, the God of Jacob …”This is my name forever; this is my title for all generations. Exo 3:15
If you listen for it, you will hear this title for God from time to time in the readings during Mass.Next Start of Story of Adam to Jesus Home