And they are not all the children of Abraham because they are by blood his descendants. No, [the promise was] Your descendants will be called and counted through the line of Isaac [though Abraham had an older son].
·and only some of Abraham’s descendants are true children of Abraham [L nor are all of Abraham’s seed/descendants (true) children]. But God said to Abraham: “·The descendants I promised you will be from Isaac [or Through Isaac your descendants will carry on your name; Gen. 21:12].”
Now this does not mean that God’s word to Israel has failed. For you cannot count all “Israelites” as the true Israel of God. Nor can all Abraham’s descendants be considered truly children of Abraham. The promise was that ‘in Isaac your seed shall be called’.
Just the fact that they come from Abraham doesn’t make them truly Abraham’s children. For the Scriptures say that the promises apply only to Abraham’s son Isaac and Isaac’s descendants, though Abraham had other children too.
Don’t suppose for a moment, though, that God’s Word has malfunctioned in some way or other. The problem goes back a long way. From the outset, not all Israelites of the flesh were Israelites of the spirit. It wasn’t Abraham’s sperm that gave identity here, but God’s promise. Remember how it was put: “Your family will be defined by Isaac”? That means that Israelite identity was never racially determined by sexual transmission, but it was God-determined by promise. Remember that promise, “When I come back next year at this time, Sarah will have a son”?
Being descendants of Abraham doesn’t make them truly Abraham’s children. For the Scriptures say, “Isaac is the son through whom your descendants will be counted,” though Abraham had other children, too.
Nor is it as though all the banim of K’lal Yisroel are the ZERA of Avraham Avinu, but (as it is written) BEYITZCHAK YIKARE L’CHA ZERA ("In Yitzchak shall your seed be called, named, summoned" BERESHIS 21:12).