Genesis 21:9

See, I ask you, dearly beloved, in this instance once again Sarah not tolerating the brashness of Ishmael and unable to put up equably with the maidservant's son being reared with Isaac. So just as she previously wished to check Hagar's arrogance and in her extreme irritation had her sent packing, so in this case too she wanted to nip Ishmael's forwardness in the bud. Not bearing to see the son born of grace and of the very gift of God being reared along with that of the Egyptian maidservant, she said to Abraham, "Send away the maidservant and her son: surely the son of the maidservant will not share the inheritance with my son?" Since she realized that she herself was in fact in extreme old age, and she saw the patriarch was well on (after all, they were both advanced in years), she was afraid that in the event of their sudden passing Ishmael would, on the score of his being born of an association of the patriarch, endeavor to thrust himself into his father's inheritance and become a sharer of it with Isaac. Hence she said, "Send away from here the maidservant and her son." Let her learn at this stage, she is saying, that the son of the maidservant has nothing in common with my son, Isaac. It is, in fact, not fair that the slave's son should be reared with that of the mistress, my son.

Then the time came for Isaac to be born, and milk flowed in the breasts of the old woman. On the day of the great feast that Abraham prepared when he circumcised and weaned Isaac, Sarah noticed Ishmael playing.

Spiritually, therefore, all indeed who come to the recognition of God through faith can be called sons of Abraham; but among these some cling to God on the basis of love, others on the basis of dread and fear of future judgment. Whence also the apostle John says, "He who fears is not perfected in love, but perfect love casts out fear." He therefore who "is perfected in love" is born of Abraham and is "a son of the free woman." But he who keeps the commandments, not in perfect love but in dread of future torment and in fear of punishments is, indeed, also himself a son of Abraham. He too receives gifts, that is, the reward of his work (because even "he who shall give a cup of cold water only in the name of a disciple, shall not lose his reward"). Nevertheless he is inferior to that person who is perfected, not in slavish fear but in the freedom of love.The apostle also shows something similar when he says, "As long indeed as the heir is a child, he differs nothing from a servant, though he be lord of all; but he is under tutors and governors until the time appointed by the father." He is "a child," therefore, who is nourished "with milk" and "is unskillful in the word of justice"; nor is he able to receive the "solid food" of the divine wisdom and knowledge of the law.

Sarah is angry because the son of the bondwoman plays with the son of the free woman, and she considers that play to be a disaster. She counsels Abraham and says, "Cast out the bondwoman and her son. For the son of the bondwoman shall not be heir with my son, Isaac."I shall not now consider how these words ought to be understood. The apostle discussed them in this way, saying, "Tell me, you who have read the law, have you not heard the law? For it is written that Abraham had two sons, the one by a bondwoman and the other by a free woman. But he indeed who was of the bondwoman was born according to the flesh, but he of the free woman was by promise. Which things are allegorical." What then? Is Isaac not "born according to the flesh"? Did Sarah not bear him? Is he not circumcised? In regard to this very incident, that he played with Ishmael, did he not play in the flesh? This indeed is what is astonishing in the apostle's understanding, that he called things "allegorical" that are quite obviously done in the flesh. His purpose is that we might learn how to treat other passages, and especially these in which the historical narrative appears to reveal nothing worthy of the divine law. Ishmael, therefore, is born "according to the flesh," the son of the bondwoman.

And when Sarah saw the son of Hagar the Egyptian playing, she said to Abraham: "Expel this slave woman and her son, for the son of the slave woman shall not be heir with my son Isaac."