I explained to you that children born to unbelievers were not considered to be properly begotten. When Ishmael was conceived, his mohter Hagar was still a pagan. She did convert later, but there was still something wrong with Ishmael. Thus God referred to Isaac as Abraham's "only son."I think that Paul was not really referring to that when He said "they are not all Israel who are of Israel". For he went on to say:
7 nor are they all children because they are the seed of Abraham; but, “In Isaac your seed shall be called.” 8 That is, those who are the children of the flesh, these are not the children of God; but the children of the promise are counted as the seed. 9 For this is the word of promise: “At this time I will come and Sarah shall have a son.”
Paul said nothing about the things which you pointed out there. But Paul was telling us about the Israel of God, more than that they are not all who comprised the nation Israel, nor are they Israel because they are the seed of Abraham. They are the descendants of Isaac, but not really those according to the flesh, but the children of promise. So Paul here tells us that the Israel of God, are the children of promise, and these are the children of God. Paul said this as the answer, in anticipation, to those who might ask and think that the word of God has taken no effect concerning Israel, who do not believe the gospel of Jesus Christ then, who at the time even, were they who had caused the Messiah of God to be crucified.
Tong
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