Romans 9:7

Neither because they are Abraham's seed, are they all children: but in Isaac shall thy seed be called.

Abraham had many children besides Isaac, their number running perhaps into the hundreds, since he had a plurality of concubines, besides Hagar; and after Sarah's death he was married to Keturah, thought by many to have already borne the sons attributed to her, during the period of her concubinage. From whatever source, the Bible states that 318 servants were born in his house (see more on this in my Commentary on Hebrews, p. 271). At the very least, all of the sons of Keturah and Hagar were among the "sons of Abraham" but were not so reckoned among the Jews, hence the validity of Paul's reasoning here to the effect that mere fleshly connection with Abraham did not make one an Israelite.

Paul had preparing to announce God's rejection of Israel from being a favored nation, because of their rejection of Christ, and the great corollary of God's calling all people (Jews and Gentiles) into His kingdom, without regard to physical descent from Abraham; and Paul knew the vehemence with which the Jews in general would reject such an idea. He knew the grounds on which they would base their utter rejection of such a concept, the principal one being that they were the children of Abraham, to the exclusion of all others, and that they alone were heirs of the great promise to Abraham. Both Christ and John the Baptist had addressed themselves to that same adamant Jewish position. They trusted in being Abraham's seed, the Rabbis going so far as to say that no circumcised person could ever enter hell, regardless of life or character. Paul, in this verse, was showing tactfully (and tenderly, at first) that Abraham had sons, notably Ishmael, who were not regarded as the seed of Abraham, as indicated by the quotation from Genesis 21:12, "In Isaac shall thy seed be called," and thus laying down the premise that, even from the very first, it was Abraham's spiritual seed, as distinguished from his mere posterity, who were to receive the blessing and who were the legitimate heirs of the Abrahamic promise.

To the Jews of Paul's day, any suggestion to the effect that God would reject Israel would have been vociferously refused on the ground that such a rejection of themselves would have brought God's word to naught, hence Paul's introductory proposition that "It is not as though the word of God hath come to naught." Before Paul was through with this line of reasoning, he would show that, on the contrary, the word of God itself taught both the rejection of Israel and the calling of the Gentiles. The specific argument from this verse is that, just as God had rejected Ishmael who was a son of Abraham, so God was also free to reject the Jews of Paul's day (for due cause, of course), although they too are Abraham's sons (as was Ishmael), the determinator being something other than fleshly descent.