""Oh, that Ishmael were alive before you!" (Genesis 17:18). Upon hearing the news that he would have a son by his wife Sarah, Abraham asked God why he needed another son, since he already had Ishmael, the son of Hagar. What was the difference between Ishmael and Isaac? Abraham was 86 years old when he had Ishmael. For many years he had served God, overcoming numerous trials. It is understood that Abraham knew what to expect from a Jewish child. When he asked God that Ishmael be his offspring, what he meant was that he would follow in his footsteps, according to Abraham's understanding of God's path. However, God answered him that his true offspring would be from Isaac.
It is not difficult to find reasons why Abraham considered Ishmael sufficient, since, as our sages tell us, Hagar, Ishmael's mother, was an exceptional woman, to the point that her actions were deemed appropriate for the Great Temple of Jerusalem. Furthermore, she was the daughter of the king of Egypt, one of the most powerful nations on earth at that time, and she had sacrificed all the privileges of a princess to become a servant in Sarah's household. It is therefore difficult to understand why Isaac, and not Ishmael, should be Abraham's descendant.
The two main differences between Ishmael and Isaac are: 1) Ishmael was born naturally, while Isaac was born miraculously. According to the laws of nature, it was impossible for Isaac to be born in the old age of Abraham and Sarah. 2) The commandment of Brit Milah, circumcision. Ishmael was circumcised at 13 years old, when he already had a developed mind, and with it, he decided to unite with God through this commandment. Isaac was circumcised on the 8th day, when he could not ask questions, and yet, at such a young age, he was connected to God with an eternal bond.
According to these differences we will understand why God told Abraham that his true offspring would be Isaac.
A child grows up under the supervision of his parents, who take care of him in every way, educate him to have the right ideas, and, according to these ideas, the child connects with God. This is the path of Ishmael. He was raised alongside Abraham in his home, and, according to this upbringing, he had the correct understanding of how one should connect with God.
However, this knowledge and understanding is no guarantee that the person will truly connect with God, since their entire Judaism depends on the intellect. It is impossible to know how they will react to different situations throughout their life. Moreover, even without considering extreme situations, in everyday life, this person's commitment to Judaism will be limited to their intellectual grasp of it.
This is why Isaac is the true descendant of Abraham: when God set out to establish the generations of the Jewish people, it was not enough to simply follow the laws of nature. The entire existence and sustenance of the Jewish people transcends nature. The Jewish people can only arise from a child who, from birth onward, transcends nature, a child who is already connected to God at eight days old. For such a child, Judaism will not only be an intellectual matter, but an essential one.
The Torah is not a book of stories. Every event recounted has an eternal lesson.
There are those who say that a child should be raised with a "watered-down", "reduced", "modernized" Judaism and then, when he grows up, he will be given more.
This would have been correct if Judaism were something that depended purely and exclusively on the intellect. We Jews have the teaching of the story of Ishmael and Isaac. The order of true Jewish education is that a child as young as eight days old must already be given the fullest measure of Judaism, must already be connected to God with an eternal connection.
What does the highest measure of Judaism mean? It means explaining to the child that the Jewish people in general, and each Jew in particular, are not subject to nature. That one who comes from a people who, from their very beginning, have been guided by miracles, who have endured so many years of trials and tribulations, and whose very existence is solely through miracles, is not bound by nature.
Extracted from Likutei Sichot vol 1, page 18