Isaac was the child of God's promise.
When GOD told Abraham that Abraham would have a son, Sarah thought logically and gave her handmaid Hagar to Abraham as a wife. Sarah apparently thought there was no way that she would bear that child, so it would have to come from someone else. Perhaps Abraham and Sarah had been uncertain why they hadn't conceived, as to which of them (or both!) were infertile. Perhaps Sarah saw this as GOD's answer to that question. "Ah, it's me! I'm the reason we couldn't have children."
So Abraham went along with Sarah's suggestion and took Hagar as a second wife. Hagar conceived and bore a son named Ishmael.
But this wasn't the child GOD had promised. GOD had other plans.
Abraham was 86 years old when Ishmael was born. Thirteen more years passed before GOD's next revelation that the time had come to bring about His promise.
The Abraham-Sarah-Hagar relationship was rocky from the start. We aren't told what Sarah and Hagar's relationship was like before Sarah gave Hagar to Abraham as a wife. But soon thereafter, conflict arose.
Genesis 16:4-6 tells us that, once Hagar got pregnant, Hagar "despised" Sarah. Sarah, realizing her plan had backfired, appealed angrily to Abraham, and "dealt harshly" with Hagar. It was so bad that Hagar ran away.
It is at this point that we read:
7 Now the Angel of the Lord found her by a spring of water in the wilderness, by the spring on the way to Shur. 8 Then He said, “Hagar, Sarai's maid, where have you come from, and where are you going?” She replied, “I am fleeing from the presence of my mistress Sarai.”
9 The Angel of the Lord then said to her, “Return to your mistress, and humble yourself under her hand.” 10 Again, the Angel of the Lord said to her, “I will surely multiply your seed exceedingly, that it may not be counted because of its multitude.” 11 Once again, the Angel of the Lord said to her:
“Behold, you are with child,
And you shall bear a son.
You shall call his name Ishmael,
For the Lord has taken notice of your humiliation.
12 He shall be a rustic man,
And his hand shall be against every man,
And every man's hand against him.
He shall dwell in the presence of all his brethren.”
(Genesis 16:7-12 OSB)
The verbiage of GOD's promise to Hagar sounds a lot like fulfillment of GOD's promise to Abraham regarding Abraham's innumerable seed.
But that isn't the case! And this is a critical observation.
Skipping ahead a few verses, we read:
1 When Abram was ninety-nine years old, the Lord appeared to Abram and said to him, “I am your God; be pleasing before Me and blameless. 2 I will establish My covenant between Me and you, and will multiply you exceedingly.” 3 Then Abram fell on his face, and God talked with him, saying, 4 “As for Me, behold, My covenant is with you, and you shall be a father of many nations. 5 No longer shall your name be called Abram, but your name shall be Abraham; for I have made you a father of many nations. 6 I will make you exceedingly fruitful; and I will make nations of you, and kings shall come from you. 7 I will establish My covenant between Me and you and your seed after you in their generations, for an everlasting covenant, to be your God and the God of your seed after you. 8 Also I will give you and your seed after you the land you are occupying as a sojourner, all the land of Canaan, as an everlasting possession; and I will be their God.”
15 Then God said to Abraham, “As for Sarai your wife, you shall not call her name Sarai, but Sarah shall be her name. 16 And I will bless her and also give you a son by her; and I will bless him, and he shall become nations; and kings of peoples shall be from him.” 17 Then Abraham fell on his face and laughed, and said in his mind, “Shall a child be born to a man who is one hundred years old, and shall Sarah, who is ninety years old, bear a child?” 18 And Abraham said to God, “Oh, that Ishmael might live before You!” 19 Then God said, “No, Sarah your wife shall bear you a son, and you shall call his name Isaac; and I will establish My covenant with him for an everlasting covenant, and with his seed after him. 20 As for Ishmael, I have heard you. Behold, I have blessed him and will increase and multiply him exceedingly. He shall beget twelve nations, and I will make him a great nation. 21 But My covenant I will establish with Isaac, whom Sarah shall bear to you at this time next year.”
(Genesis 17:1-8, 15-21 OSB)
Descendants vs Seed
Although Ishmael would have innumerable descendants, it would not be through Ishmael that GOD's promises to Abraham would be fulfilled.
Instead, the fulfillment would come through Isaac.
There is a critical distinction between descendants and children of promise. We must recognize this distinction or we will misunderstand subsequent biblical prophecy.
Abraham asked GOD that the promise might be fulfilled through Ishmael, but GOD said no. GOD would continue His covenant through Isaac.
The apostle Paul wrote about this distinction several times. For example:
1 I speak the truth in Christ—I am not lying, my conscience confirms it through the Holy Spirit— 2 I have great sorrow and unceasing anguish in my heart. 3 For I could wish that I myself were cursed and cut off from Christ for the sake of my people, those of my own race, 4 the people of Israel. Theirs is the adoption to sonship; theirs the divine glory, the covenants, the receiving of the law, the temple worship and the promises. 5 Theirs are the patriarchs, and from them is traced the human ancestry of the Messiah, who is God over all, forever praised! Amen.
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.”
(Romans 9:1-8 NIV)
To paraphrase, while Abraham had many physical descendants, the promises GOD made to Abraham were realized only through Isaac (and then Jacob, aka Israel) and then the faithful descendants of Israel. GOD made promises to Abraham because He knew that Abraham had faith. Isaac had faith. Jacob / Israel had faith. And while GOD made a covenant with the children of Israel, most of them were not faithful—they were not GOD's children.
Today, it is faith, not physical lineage, that determines whether one is a Jew/Israelite. Paul wrote:
28 A person is not a Jew who is one only outwardly, nor is circumcision merely outward and physical. 29 No, a person is a Jew who is one inwardly; and circumcision is circumcision of the heart, by the Spirit, not by the written code. Such a person’s praise is not from other people, but from God.
(Romans 2:28-29 NIV)
In Galatians, Paul contrasted the Hagar-Sarah descendants as an allegory of the difference between the first covenant with Israel (at Sinai) and the second covenant with Israel (through Jesus):
21 Tell me, you who want to be under the law, are you not aware of what the law says? 22 For it is written that Abraham had two sons, one by the slave woman and the other by the free woman. 23 His son by the slave woman was born according to the flesh, but his son by the free woman was born as the result of a divine promise.
24 These things are being taken figuratively: The women represent two covenants. One covenant is from Mount Sinai and bears children who are to be slaves: This is Hagar. 25 Now Hagar stands for Mount Sinai in Arabia and corresponds to the present city of Jerusalem, because she is in slavery with her children. 26 But the Jerusalem that is above is free, and she is our mother. 27 For it is written:
“Be glad, barren woman,
you who never bore a child;
shout for joy and cry aloud,
you who were never in labor;
because more are the children of the desolate woman
than of her who has a husband.”
28 Now you, brothers and sisters, like Isaac, are children of promise. 29 At that time the son born according to the flesh persecuted the son born by the power of the Spirit. It is the same now. 30 But what does Scripture say? “Get rid of the slave woman and her son, for the slave woman’s son will never share in the inheritance with the free woman’s son.” 31 Therefore, brothers and sisters, we are not children of the slave woman, but of the free woman.
(Galatians 4:21-31 NIV)
GOD's eternal covenant and blessings have always been about faith, not physical birth. Yes, the first covenant with Israel at Sinai was one of physical lineage, but GOD rejected and punished and destroyed many of those people because they lacked faith. Paul wrote:
1 For I do not want you to be ignorant of the fact, brothers and sisters, that our ancestors were all under the cloud and that they all passed through the sea. 2 They were all baptized into Moses in the cloud and in the sea. 3 They all ate the same spiritual food 4 and drank the same spiritual drink; for they drank from the spiritual rock that accompanied them, and that rock was Christ. 5 Nevertheless, God was not pleased with most of them; their bodies were scattered in the wilderness.
6 Now these things occurred as examples to keep us from setting our hearts on evil things as they did. 7 Do not be idolaters, as some of them were; as it is written: “The people sat down to eat and drink and got up to indulge in revelry.” 8 We should not commit sexual immorality, as some of them did—and in one day twenty-three thousand of them died. 9 We should not test Christ, as some of them did—and were killed by snakes. 10 And do not grumble, as some of them did—and were killed by the destroying angel.
11 These things happened to them as examples and were written down as warnings for us, on whom the culmination of the ages has come. 12 So, if you think you are standing firm, be careful that you don’t fall!
(1 Corinthians 10:1-12 NIV)
So, as you read the Bible and you come across promises and prophecy regarding eternal or heavenly blessings, remember that these promises were always—and forever will be always—for those who have faith.
To put it bluntly, there are no authentic Mosaic Jews today because no one can trace their lineage back to Jacob, there is no Levitical priesthood, and there is no temple in Jerusalem. Nor does GOD have an interest in Christians today obeying the Mosaic Law or Israelite customs.