Albert Barnes Commentary Romans 9:8

"That is, it is not the children of the flesh that are children of God; but the children of the promise are reckoned for a seed." — Romans 9:8 (ASV)

They which are the children of the flesh. The natural descendants.

These are not the children of God. Are not necessarily the adopted children of God, nor are they so simply by virtue of their descent. This was in opposition to one of the most settled and deeply-cherished opinions of the Jews. They supposed that the mere fact of being a Jew entitled a person to the blessings of the covenant and to be regarded as a child of God. But the apostle shows them that these spiritual privileges were not granted by their natural descent. He demonstrates that they were not conferred on people simply from the fact that they were Jews. Consequently, those who were not Jews might also share in these spiritual blessings.

But the children of the promise. The descendants of Abraham on whom the promised blessings would be bestowed. The meaning is that God, at first, contemplated a distinction among the descendants of Abraham and intended to confine His blessings to those whom He chose—that is, to those to whom the promise particularly pertained, the descendants of Isaac. The apostle's argument is that the principle was thus established: a distinction might be made among those who were Jews. And, as that distinction had been made in former times, so it might also be under the Messiah.

Are counted. Are regarded, or reckoned. God reckons things as they are and therefore intended that they should be His true children.

As the seed. The spiritual children of God; the partakers of His mercy and salvation. This undoubtedly refers to spiritual privileges and to salvation, and therefore relates not to nations as such, but to individuals.