Romans 9:8

“That is, they which are the children of the flesh, etc..] This is an explanation of the foregoing verse, and shows, that by “the seed of” Abraham are meant, the natural seed of Abraham, who are born after the flesh, or descend from him by carnal generation: these are not the children of God; that is, not all of them, nor any of them, on account of their being children of the flesh, or Abraham's natural seed; for adoption does not come this way; men do not commence children of God by their fleshly descent; they are not “born of blood”, but of God, who are the sons of God: but the children of the promise are counted for the seed; tyrb ynb “children of the covenant”, is a common phrase with the…”

“They which are the children of the flesh - The natural descendants. These are not the children of God - Are not of necessity the adopted children of God; or are not so in virtue of their descent merely. 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 man to the blessings of the covenant, and to be regarded as a child of God. But the apostle shows them that it was not by their natural descent that these spiritual privileges were granted; that they were not conferred on people simply from the fact that they were Jews; and that consequently those who were not Jews might become interested in…”

“That is, Not the children, &c. - As if he had said, This is a clear type of things to come; showing us, that in all succeeding generations, not the children of the flesh, the lineal descendants of Abraham, but the children of the promise, they to whom the promise is made, that is, believers, are the children of God. Gen 21:12 Romans 9:9”

“They which: Rom 4:11-16; Gal 4:22-31 are counted: Gen 31:15; Psa 22:30, Psa 87:6; Joh 1:13; Gal 3:26-29, Gal 4:28; Jo1 3:1, Jo1 3:2 Romans 9:9”