6 But it is not that the word of God has taken no effect. For they are not all Israel who are of Israel, 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.”"For they are not all Israel who are of Israel."
This is a play on words in which Paul uses the word "Israel" in two senses ... one as a proper name (the first) and one in its meaning (the second). Paul speaks of Israel as his countrymen in general. But not all of these are "of Israel." The Hebrew word "yiśrâ'êl" means "he will rule as God." More simply put, "All of this nation will not rule as God." This is still pretty basic. He says that this is butressed by the fact that merely being a result of Abraham mating with a woman was not the being "the seed" ... because Abraham had offspring from other than Isaac. (Ishmael, etc) The "seed of Abraham" in the biblical sense were the physical seed, but it was that which was promised. (Not the creative engineering of Sarah re: Hagar) The physicality of this can't be denied, but it is not merely physical.
Help out if you can, but try not to jump ahead.