|… And he said, Take now thy son, thine only son Isaac, whom thou lovest, and get thee
into the land of Moriah; and offer him there for a burnt offering upon one …
//biblehub.com/genesis/22-2.htm – 19k
|… thine hand upon the lad, neither do thou any thing unto him: for now I know that
thou fearest God, seeing thou hast not withheld thy son, thine only son from me …
//biblehub.com/genesis/22-12.htm – 18k
… And said, By myself have I sworn, saith the LORD, for because thou hast done
this thing, and hast not withheld thy son, thine only son: …
//biblehub.com/genesis/22-16.htm – 17k
According to Islam this is because the evil Jews tried to take Ishmael, the firstborn son of Abraham, out of the covenant and replace him with Isaac. There is never a fanciful conspiracy theory far away from the religion of Islam.
If so it would be a very clumsy attempt because everybody knows that Ishmael was the firstborn son of Abraham because the bible testifies to this:
Genesis 16 v 15: And Hagar bare Abram a son: and Abram called his son’s name, which Hagar bare, Ishmael.
So why would the evil Jews, who corrupted their scriptures and apostazised ungratefully according to the Koran, leave this text in if they wanted to hide the fact that Ishmael was the firstborn son of Abraham and the rightful heir?
The problem for Islam is that Jehovah chooses whom he wills to be the heir of his promises and it wasn’t Ishmael that he chose as the text shows. We know that Ishmael had no share in the promised land. It was given by Jehovah to the twelve tribes of Israel. So he could not have been the one through whom the promise of blessing to the whole world would come and Jehovah confirms this.
My own belief is that Isaac is called the ONLY SON of Abraham because of his miraculous origin. It may be the case that Ishmael was not a legitimate son. However it does say that Sarah gave Hagar to Abraham to be his wife. So for argument’s sake I am willing to concede that Hagar was the wife of Abraham. They were one flesh before God. However just as Esau was rejected so Ishmael was also rejected by God to be the bearer of the promise. God is sovereign. He also chose David over all his older brothers to be the king of Israel.
To recap, Isaac is called the ONLY SON of Abraham because he was not born solely by natural means. His mother was infertile and probably Abraham also. So Isaac was a unique son of Abraham and he was the type of Jesus who also was not a natural human being in terms of his origin.