Bede : For the son of the bondwoman shall not be heir with my son Isaac. Which the Apostle states as with the son of the freewoman, because in no way should false teachers, who affirm that this grace benefits us only if we are also consecrated by circumcision according to the rite of the law, be received with the preachers of the truth, who proclaim that we should be saved by the grace of the Lord Jesus. Rather, they shall remain forever excluded from the inheritance of grace, which is in Christ, those who deny the power of the same grace, or who exalt themselves by works without grace. There are also some today in the Church, indeed initiated into the sacraments of the new testament, but through the intention of a carnal mind pertaining to the old testament and to the figure of Hagar and Ishmael; not that they truly follow the commandments of the old testament, of which the Lord says: If you want to enter into life, keep the commandments (Matthew XIX, 17), but because they seek temporal benefits from the Lord, neglecting eternal ones, which the old Scripture understood according to the letter sometimes seems to sound, embracing more what the Lord says through the Prophet: If you are willing and obedient, you shall eat the good of the land (Isaiah I, 19) rather than what He says through Himself: Blessed are the poor, for yours is the kingdom of God (Matthew V); concerning whom the Apostle says: For all seek their own, not the things which are of Jesus Christ (Philippians II, 21), who either continue in wicked deeds among good Catholics till death, or because of heresies and obvious schisms are expelled from the Church by the judgment of the priest as the sons of the bondwoman through the free Sarah, all the same, in the future judgment, who have not previously corrected themselves, will be rendered and excluded from the inheritance of blessing: For everyone who commits sin is a slave of sin; the slave does not stay in the house forever; the son remains forever. If therefore the son sets you free, you shall be free indeed (John VIII, 34). — Commentary on Genesis (Hexaemeron) Ephrem the Syrian : Then Sarah, who showed no envy in any matter that concerned herself, became envious in this matter concerning her son. She was not envious of Hagar whom she had given to her husband. Since it was a matter of God's promise, and the son of the concubine thought that he would be coheir with the son of the freewoman, Sarah said, "Cast out the slave woman and her son, because it is not just that a son of a handmaid should have any inheritance together with that son of the promise, to whom it was promised by God. It is not right that you be opposed to God and make an heir him whom God has not made an heir." — COMMENTARY ON Genesis 18:2 Augustine of Hippo : ON THESE PROPHETIC WORDS: CAST OUT THIS MAID AND HER SON, ETC. — The question arises as to why Abraham is saddened to hear the words of Sarah: Cast that slave and her son, for the son of that slave woman is not to be heir with my son Isaac, being that he obviously had to know better than Sarah that this was a prophecy (Cf. Gal. 4:21). So we have to think that Sarah said this by revelation, inasmuch as it had been revealed first to her, and that Abraham, knowing it later, because God made it known to him, was shaken, full of paternal affection, by the luck of his son; or that both were ignorant at first of the meaning of that, and Sarah, who was ignorant of it, said it prophetically, moved by her feminine animosity because of the pride of the slave. [ — QUESTIONS ON GENESIS, Q. 50 (tr. J. Litteral, draft)