Charles Ellicott 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.—The scholar will observe that there appears to be a mixture of two constructions here: “the case is not such that,” and “I do not mean to say that” (or “I do not intend to say that the case is such as that”).

Taken none effect.—“Fallen through,” or “failed to be accomplished.”

Of Israel—i.e., descended from Jacob. (Compare Genesis 32:28.) The promise of God was indeed given to Israel, but that did not simply mean all who could claim descent from Jacob without further limitation.

On verses 6-13:

Now follows a vindication of God’s dealings in rejecting Israel. And this is divided into three parts. Part 1 extends to the end of Romans 9:13, and its object is to clear the way by defining the true limits of the promise. It was not really to all Israel that the promise was given, but only to a particular section of Israel.