Augustine of Hippo
Catena Aurea · Johannesevangelium 3
Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother’s womb, and be born? Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God. That which is born of the flesh is flesh; and that which is born of the Spirit is spirit. Marvel not that I said unto thee, Ye must be born again. The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth; so is every one that is born of the Spirit.
(Tr. xii. c. 5) As if He said, Thou understandest me to speak of a carnal birth; but a man must be born of water and of the Spirit, if he is to enter into the kingdom of God. If to obtain the temporal inheritance of his human father, a man must be born of the womb of his mother; to obtain the eternal inheritance of his heavenly Father, he must be born of the womb of the Church. And since man consists of two parts, body and soul, the mode even of this latter birth is twofold; water the visible part cleansing the body; the Spirit by His invisible cooperation, changing the invisible soul.