Romans 9:9

For this is a word of promise, According to this season will I come, and Sarah shall have a son.

In distinction from all the other sons of Abraham, Isaac was the child of promise; and Paul here left nothing unsaid with reference to it, citing the very passage that recorded God's promise (Genesis 18:10). Now, Christ is the antitype of Isaac (my Commentary on Hebrews, p. 277); and therefore Jesus Christ (along with the spiritual seed who are "in him") has the same preference over all the fleshly descendants of Abraham that Isaac had over his fleshly brothers. God's righteousness, the great theme of Romans, was ever before Paul's mind; and his purpose in these verses was to show that God's actions in the calling of the Gentiles and rejection of Israel were in no degree blameworthy, but righteous. Even the rejection of Israel as a favored nation and the admission of Gentiles to the kingdom of God did not, in any sense whatever, exclude Jews, the only injury to them in such actions being the destruction of their sinful pride. All of the marvelous blessings of the kingdom of Christ were available to all Jews and Gentiles alike, without preference, and upon the same terms; and the blessings and privileges of the new kingdom were far superior in every way to anything the Jews had enjoyed under the old system.