Adam Clarke on Romans 9:8

That is, They which are the children of the flesh - Whence it appears that not the children who descend from Abraham's loins, nor those who were circumcised as he was, nor even those whom he might expect and desire, are therefore the Church and people of God; but those who are made children by the good pleasure and promise of God, as Isaac was, are alone to be accounted for the seed with whom the covenant was established.