God’s purpose is not utterly defeated by this infidelity

^6-7 Now this does not mean that God’s word to Israel has failed. For you cannot count all “Israelites” as the true Israel of God. Nor can all Abraham’s descendants be considered truly children of Abraham. The promise was that ‘in Isaac your seed shall be called’.

^8-12 That means that it is not the natural descendants who automatically inherit the promise, but, on the contrary, that the children of the promise (i.e. sons of God) are to be considered truly Abraham’s children. For it was a promise when God said: ‘At this time I will come and Sarah shall have a son’. (Everybody, remember, thought it quite impossible for Sarah to have a child.) And then, again, a word of promise came to Rebecca, at the time when she was pregnant with two children by the one man, Isaac our forefather. It came before the children were born or had done anything good or bad, plainly showing that God’s act of choice has nothing to do with achievements, good or bad, but is entirely a matter of his will. The promise was: ‘The older shall serve the younger’.