Matthew Poole and continuators on Romans 9:8

8. That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed .

q. d. That I may speak more plainly, all those that are the Children of Abraham according to the Flesh, are not therefore the Adopted Children of God; it is not their Blood, but their Faith must make them such. There are some of Abraham's Seed, that are Selected from the rest, to whom the Promise was made; who are therefore called children of the Promise: And of this sort are all they, who are Born after the Spirit, (as Isaac is said to be, Gal. 4.23.) whether Iews or Gentiles. The Sence of this Verse is fully exprest, Gal. 3.8, 14. and 3.29. vid. Gal. 4.28.