Romans 9:8

In this verse Paul detaches the general principle from the particular fact which has just been cited. The τουτέστι, that is, exactly expresses his intention to derive from the historical fact the principle on which it rests. Ishmael's birth proceeded from the flesh, that is to say, had nothing in it except what was human. In Isaac's, God interposed with his promise; and it was from this divine promise, according to chap. 4, that Abraham by faith drew the strength which rendered him capable of becoming father of the promised seed. In consequence of this higher element, only Isaac and his descendants can be regarded as God's children. This is what explains the second proposition of the verse, in which the name of the (promised) seed is expressly given to the descendants obtained by faith in the promise.

The first proposition of this verse implicitly legitimates the rejection of the Jews according to the flesh; the second, the adoption of the believing Gentiles.