Albert Barnes Commentary Romans 9:6

"But [it is] not as though the word of God hath come to nought. For they are not all Israel, that are of Israel:" — Romans 9:6 (ASV)

Not as though, etc. Not as though the promise of God had entirely failed. Though I grieve so, (Romans 9:2–3) though I am deeply apprehensive for the nation, yet I do not affirm that all the nation is to be destroyed. The promise of God will not entirely fail.

Not all Israel. Not all the descendants of Jacob have the true spirit of Israelites, or are Jews in the scriptural sense of the term. See Barnes on Romans 2:28-29.