Charles Spurgeon Commentary Romans 9:6-7

"But [it is] not as though the word of God hath come to nought. For they are not all Israel, that are of Israel: neither, because they are Abraham`s seed, are they all children: but, In Isaac shall thy seed be called." — Romans 9:6-7 (ASV)

For they are not all Israel, which are of Israel: Neither, because they are the seed of Abraham, are they all children:

Now he goes on to show that the blessings of God's grace do not go according to carnal descent. It is true that God promised to bless the seed of Abraham, yet he meant that word "seed" in a very special sense.

Not as though the word of God has taken no effect. For they are not all Israel, which are of Israel: Neither, because they are the seed of Abraham, are they all children: but, in Isaac shall your seed be called.

Now, the Apostle is getting to his point. You Jews claim to have the mercy of God because you are of the seed of Abraham; but there is nothing in that, he says, for God made a distinct choice of Isaac to the rejection of Ishmael, as he did afterwards of Jacob, and then Esau was left out.