The Apostle Paul in Romans 9 is dealing with the question: "If Israel - Abraham's seed - are God's chosen people, why have so many Jews rejected Jesus? In light of the Jews' rejection of Jesus, Paul anticipates the question: "Has the word of God failed" because God promised to save the Jews and yet the Jews have not been saved?

Paul's answer is "they are not all (true) Israel who are of (ethnic) Israel" (Romans 9:6). God never promised to save ALL of ethnic Israel, therefore His word has NOT failed.

But then what determines who among ethnic Israel is the true (or spiritual) Israel? Who among ethnic Israel are "the children of God"? The answer: those who are "the children of the promise."