Romans 9:6:
It is not as though God’s word had failed. For not all who are descended from Israel are Israel. (NIV)
But it is not as though the word of God has failed For they are not all Israel who are descended from Israel; (NASB)
Not as though the word of God hath taken none effect. For they are not all Israel, which are of Israel: (KJV)
Here, Paul replies to a prolepsis arising fromm Rom 9:1–5 in regards to the discourse in Rom 1–8.
The argument against Paul’s statements: “In God’s word to Jews are the privileges of adoption as sons, the divine glory, the covenants, the receiving of the law, the temple worship, the promises, theirs are the patriarchs, from their patriarchs is traced the human ancestry of Christ (Rom 9:3–5); ‘Now a righteousness from God, apart from law, has been made known’ (Rom 3:21); therefore, because of the Jews who will not enjoy the privileges in God’s word, God’s word failed.”
Paul refutes this (“It is not as thought God’s word has failed (Rom 9:6)”) with an argument: “In God’s word to Jews are the privileges of adoption as sons, the divine glory, the covenants, the receiving of the law, the temple worship, the promises, theirs are the patriarchs, from their patriarchs is traced the human ancestry of Christ (Rom 9:3–5); ‘Not all who are descended from Israel are Israel’ (Rom 9:6); therefore, despite the Jews who will not enjoy the privileges in God’s word, God’s word has not failed.”
A similar argument has already been made in Rom 2:28–29: “A man is not a Jew if he is only one outwardly, nor is circumcision merely outward and physical. No, a man is a Jew if he is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a man’s praise is not from men, but from God.” (NIV)
Paul makes it clear that God’s word has not, nor will be, nullified. He similarly addresses such a prolepsis in Rom 3:3–4: “What if some did not have faith? Will their lack of faith nullify God’s faithfulness? Not at all! Let God be true, and every man a liar. As it is written: ‘So that you may be proved right when you speak and prevail when you judge.’” (NIV)