Ishmael was never considered a son by God.
Many Christian scholars refer to Ishmael as an ‘illegitimate child’, but this is not a fair or biblically accurate statement. The scriptures state that Hagar was given to Abraham to be his wife. Out of this union, Ishmael was born, whom Abraham naturally referred to as a son (as we all would). However, God refers to Ishmael as ‘the boy’, ‘descendant’ and ‘son of the slave’. He does not call Ishmael a son of Abraham. He specifically refers to Ishmael as a descendant of Abraham and never refers to Ishmael as Abraham’s son.
Genesis 21:11-13 – The matter distressed Abraham greatly because of his son Ishmae
l. 12 But God said to Abraham, “Do not be distressed because of the boy and your slave woman; whatever Sarah tells you, listen to her, for through Isaac your descendants shall be named. 13 And of the son of the slave woman I will make a nation also, because he is your descendant.
God refers to Ishmael only as the slave woman’s son and as Abraham’s descendant, even though
biologically, Ishmael is in all pretenses and purposes, Abraham’s son.
It is
Isaac whom God refers to as Abraham’s son – his
only son.
Genesis 22:2 – Then He said, “Take now your son, your only son, whom you love, Isaac, and go to the land of Moriah, and offer him there as a burnt offering on one of the mountains of which I will tell you.”
Genesis 22:12 – He said, “Do not reach out your hand against the boy, and do not do anything to him; for now I know that you fear God, since you have not withheld your son, your only son, from Me.”
Genesis 22:16 – and said, “By Myself I have sworn, declares the Lord, because you have done this thing and have not withheld your son, your only son…
And so we can see that in God’s eyes, a son is different from a descendant. Ishmael and Isaac were both biological sons of Abraham, but God considered Ishmael merely a descendant, meaning only biological. It was Isaac, who represented the promises given to Abraham. Isaac believed in the promises of God like his father, Abraham, without seeing any of it come to pass during his lifetime, and was considered a son. Isaac believed without seeing, like Abraham did, making him Abraham’s spiritual (and biological) son. He had the faith of Abraham.
The Apostle Paul also explains this in:
Romans 9:6-9 – 6 It is not as though God’s word had failed. For not all who are descended from Israel are Israel. 7 Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.” 8 In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring. 9 For this was how the promise was stated: “At the appointed time I will return, and Sarah will have a son.”