{But it is not as though} (ouch hoion de hoti). Supply
estin after ouch: "But it is not such as that," an old idiom,
here alone in N.T. {Hath come to nought} (ekpept(936b)en). Perfect
active indicative of ekpipt(935c), old verb, to fall out. {For they
are not all Israel, which are of Israel} (ou gar pantes hoi ex
Isra(886c) houtoi Isra(886c)). "For not all those out of Israel (the
literal Jewish nation), these are Israel (the spiritual Israel)."
This startling paradox is not a new idea with Paul. He had
already shown ( Ga 3:7-9 ) that those of faith are the true sons
of Abraham. He has amplified that idea also in  Ro 4 . So he is
not making a clever dodge here to escape a difficulty. He now
shows how this was the original purpose of God to include only
those who believed. {Seed of Abraham} (sperma Abraam). Physical
descent here, but spiritual seed by promise in verse  8 . He
quotes  Ge 21:12f .