2 Spiritual regeneration is one, just as the generation of the flesh is one. And Nicodemus said the truth when he said to the Lord that a man cannot, when he is old, return again into his mother's womb and be born. He indeed said that a man cannot do this when he is old, as if he could do it even were he an infant. But be he fresh from the womb, or now in years, he cannot possibly return again into the mother's bowels and be born. But just as for the birth of the flesh, the bowels of woman avail to bring forth the child only once, so for the spiritual birth the bowels of the Church avail that a man be baptized only once.
Therefore, in case one should say, “Well, but this man was born in heresy, and this in schism:” all that was cut away, if you remember what was debated to you about our three fathers, of whom God willed to be called the God, not that they were thus alone but because in them alone the figure of the future people was made up in its completeness. For we find one born of a bond woman disinherited, one born of a free woman made heir: again, we find one born of a free woman disinherited, one born of a bond woman made heir.
Ishmael, born of a bond woman, disinherited; Isaac, born of a free woman, made heir: Esau, born of a free woman, disinherited; the sons of Jacob, born of bond women, made heirs. Thus, in these three fathers the figure of the whole future people is seen: and not without reason God says, “I am the God of Abraham, and the God of Isaac, and the God of Jacob: this,” says He, “is my name for ever.” Rather let us remember what was promised to Abraham himself: for this was promised to Isaac, and also to Jacob.
What do we find? “In your seed shall all nations be blessed.” At that time the one man believed what as yet he saw not: men now see, and are blinded. What was promised to the one man is fulfilled in the nations; and they who will not see what is already fulfilled, are separating themselves from the communion of the nations. But what avails it them that they will not see? See they do, whether they will or no; the open truth strikes against their closed eyes.
Source: Tractates on the Gospel of John (New Advent)