12 A great mystery is there. They were playing together, Ishmael and Isaac: Sarah sees them playing, and says to Abraham, “Cast out the bond woman and her son; for the son of the bond woman shall not be heir with my son Isaac.” And when Abraham was sorrowful, the Lord confirmed to him the saying of his wife. Now here is evidently a mystery, that the event was somehow pregnant with something future. She sees them playing, and says, “Cast out the bond woman and her son.”
What is this, brethren? For what evil had Ishmael done to the boy Isaac, in playing with him? That playing was a mocking; that playing signified deception. Now attend, beloved, to this great mystery. The apostle calls it persecution; that playing, that play, he calls persecution: for he says, “But as then he that was born after the flesh, persecuted him that was born after the Spirit, so also now;” that is, they that are born after the flesh persecute them that are born after the Spirit.
Who are born after the flesh? Lovers of the world, lovers of this life. Who are born after the Spirit? Lovers of the kingdom of heaven, lovers of Christ, men that long for eternal life, that worship God freely. They play, and the apostle calls it persecution. For after he said these words, “And as then he that was born after the flesh persecuted him that was born after the Spirit, so also now;” the apostle went on, and showed of what persecution, he was speaking: “But what says the Scripture?
Cast out the bond woman and her son: for the son of the bond woman shall not be heir with my son Isaac.” We search where the Scripture says this, to see whether any persecution on Ishmael's part against Isaac preceded this; and we find that this was said by Sarah when she saw the boys playing together. The playing which Scripture says that Sarah saw, the apostle calls persecution. Hence, they who seduce you by playing, persecute you the more. “Come,” say they, “Come, be baptized here, here is true baptism for you.”
Do not play, there is one true baptism; that other is play: you will be seduced, and that will be a grievous persecution to you. It were better for you to make Ishmael a present of the kingdom; but Ishmael will not have it, for he means to play. Keep your father's inheritance, and hear this: “Cast out the bond woman and her son; for the son of the bond woman shall not be heir with my son Isaac.”
Source: Tractates on the Gospel of John (New Advent)