4 No man can return into his mother's bowels and be born again. But some one is born of a bond woman? Well, did they who were born of bond women at the former time, return into the wombs of the free to be born anew? The seed of Abraham was in Ishmael also; but that Abraham might have a son of the bond maid, it was at the advice of his wife. The child was of the husband's seed, not of the womb, but at the sole pleasure of the wife. Was his birth of a bond woman the reason why he was disinherited?
Then, if he was disinherited because he was the son of a bond woman, no sons of bond women would be admitted to the inheritance. The sons of Jacob were admitted to the inheritance; but Ishmael was put out of it, not because born of a bond woman, but because he was proud to his mother, proud to his mother's son; for his mother was Sarah rather than Hagar. The one gave her womb, the other's will was added: Abraham would not have done what Sarah willed not: therefore was he Sarah's son rather.
But because he was proud to his brother, proud in playing, that is, in mocking him; what said Sarah? “Cast out the bond woman and her son; for the son of the bond woman shall not be heir with my son Isaac.” It was not, therefore, the bowels of the bond woman that caused his rejection, but the slave's neck. For the free-born is a slave if he is proud, and, what is worse, the slave of a bad mistress, of pride itself. Thus, my brethren, answer the man, that a man cannot be born a second time; answer fearlessly, that a man cannot be born a second time.
Whatever is done a second time is mockery, whatever is done a second time is play. It is Ishmael playing, let him be cast out. For Sarah observed them playing, says the Scripture, and said to Abraham, “Cast out the bond woman and her son.” The playing of the boys displeased Sarah. She saw something strange in their play. Do not they who have sons like to see them playing? She saw and disapproved it. Something or other she saw in their play; she saw mockery in it, observed the pride of the slave; she was displeased with it, and she cast him out.
The children of bond women, when wicked, are cast out; and the child of the free woman, when an Esau, is cast out. Let none, therefore, presume on his birth of good parents; let none presume on his being baptized by holy men. Let him that is baptized by holy men still beware lest he be not a Jacob, but an Esau. This would I say then, brethren, it is better to be baptized by men that seek their own and love the world, which is what the name of bond woman imports, and to be spiritually seeking the inheritance of Christ, so as to be as it were a son of Jacob by a bond woman, than to be baptized by holy men and to become proud, so as to be an Esau to be cast out, though born of a free woman.
Hold ye this fast, brethren. We are not coaxing you, let none of your hope be in us; we flatter neither ourselves nor you; every man bears his own burden. It is our duty to speak, that we be not judged unhappily: yours to hear, and that with the heart, lest what we give be required of you; nay, that when it is required, it may be found a gain, not a loss.
Source: Tractates on the Gospel of John (New Advent)