Shmuel Feldman - Yitzchak
We read in Genesis (17:7–27) how G‑d appears to Abraham and instructs him to circumcise himself and all the males of his household. G‑d further commands that henceforth every newborn male should be circumcised on the eighth day of his life, as a sign of the “eternal covenant” between G‑d and the seed of Abraham.
G‑d then informs Abraham that, in one year’s time, he and Sarah will have a son, Isaac. Abraham was nearing his hundredth year at the time, and Sarah was approaching the age of ninety; the two had been married for 75 childless years, and Sarah was physically incapable of having children. Abraham already had a son, Ishmael, born thirteen years earlier, after Sarah had urged him to marry her maidservant Hagar, so that he could father a child through her.
Abraham’s reaction to the divine promise was to proclaim, “If only Ishmael would live before You!” Abraham seems to be saying that he would be perfectly happy to see Ishmael as his heir—as the one who continues his life’s work and perpetuates his special relationship with G‑d.
G‑d rejects Abraham’s proposal. He reassures him that Ishmael will become a great people, “but my covenant I shall establish with Isaac.” Only Isaac, the son you will have with Sarah, can be your true heir, and only Isaac can father the people with whom I will enter into a covenant as my “kingdom of priests and holy nation.”
This is more than a technical choice. G‑d’s insistence on Isaac as the progenitor of His chosen people tells us something very fundamental about the nature of our relationship with Him.
For Ishmael and Isaac differed in two significant respects:
Ishmael came into the world by natural means, while Isaac’s birth was a supernatural event.
Ishmael was circumcised at the age of thirteen, the age of da'at (awareness), whereas Isaac entered into the covenant of circumcision as an eight-day-old infant—an age at which a person is not even aware of what is taking place, much less of its significance.
In other words, Ishmael represents a rational relationship with G‑d, one that is based upon a person’s nature and understanding. Isaac represents a supernatural, supra-rational bond.
Abraham discerned many positive qualities in Ishmael and was prepared, and even desirous, to see him as his heir. Yet G‑d insisted that his covenant with Abraham be perpetuated specifically through Isaac and Isaac’s descendants—a people whose commitment to G‑d will transcend the natural and the rational.