And, behold, the word of the LORD came unto him, saying,
This shall not be your heir; but he that shall come forth out of your own body shall be your heir.
Read Chapter 15
Ambrose of Milan
But if the words of Abraham are not enough to correct, consider the word of God, who condemns such a mode of transmitting inheritance. “This man shall not be your heir,” he says, “but the other who will come out from you, he will be your heir.” Who is this other of whom he speaks? In fact Hagar too bore a son, Ishmael, but he is not speaking of him. Instead, he is speaking of holy Isaac. For this reason he added “who will come out from you.” In fact, the one who truly came out of Abraham is the one who was born of a legitimate marriage. But in Isaac, the legitimate son, we can see the One who is the true legitimate son, the Lord Jesus, of whom at the beginning of the Gospel according to Matthew we read that he is the son of Abraham. He was the true heir of Abraham, bringing renown to the descendants of the progenitor. Through him Abraham looked up to heaven and understood that the splendor of his posterity would be no less luminous than the radiance of the stars of heaven. As “one star differs from another in brightness, so it is also for the resurrection of the dead,” said the apostle. The Lord, in joining to his resurrection people whom death was accustomed to hide in the ground, made them sharers in the heavenly kingdom. .