Why could Ishmael not be heir along with Isaac?
Ishmael could not be heir with Isaac because he was the son of the bondwoman Hagar, born after the flesh, while Isaac was the son of promise. Scripture declares the son of the bondwoman shall not be heir with the son of promise, even with Isaac.
Les explains that Isaac and Ishmael had to be separated because Ishmael was the son of the bondwoman Hagar, representing the flesh, while Isaac was the son of promise. In Genesis 21, Sarah saw Hagar's son mocking and told Abraham to cast out the bondwoman and her son, declaring that the son of the bondwoman would not be heir with her son, even with Isaac. Les points out that Abraham loved Ishmael, but he lacked the long-range concept of the Abrahamic covenant—that the Messiah would come through his loins. God told Abraham to hearken to Sarah. Earlier, Les shows that the singular seed refers to those of the promise, not the flesh.
Wherefore she said unto Abraham, Cast out this bondwoman and her son: for the son of this bondwoman shall not be heir with my son, even with Isaac.
That is, They which are the children of the flesh, these are not the children of God: but the children of the promise are counted for the seed.
More Questions
Why did Sarah insist that Hagar and Ishmael be cast out according to Genesis 21:10?
Sarah demanded that Abraham cast out the bond woman and her son because the son of the bond woman would not be heir with Isaac, the son of promise.
Why did Sarah tell Abraham to cast out Hagar and Ishmael?
Sarah demanded that Hagar and Ishmael be cast out because Ishmael was taunting young Isaac, and 90-year-old Sarah could not tolerate it.