6 This Nicodemus, who had come to Jesus by night, did not savor of this spirit and this life. Says Jesus to him, “Unless a man be born again, he shall not see the kingdom of God.” And he, savoring of his own flesh, while as yet he savored not of the flesh of Christ in his mouth, says, “How can a man be born a second time, when he is old? Can he enter a second time into his mother's womb, and be born?” This man knew but one birth, that from Adam and Eve; that which is from God and the Church he knew not yet: he knew only those parents that bring forth to death, knew not yet the parents that bring forth to life; he knew but the parents that bring forth successors, knew not yet the ever-living parents that bring forth those that shall abide.
Whilst there are two births, then, he understood only one. One is of the earth, the other of heaven; one of the flesh, the other of the Spirit; one of mortality, the other of eternity; one of male and female, the other of God and the Church. But these two are each single; there can be no repeating the one or the other. Rightly did Nicodemus understand the birth of the flesh; so understand also the birth of the Spirit, as Nicodemus understood the birth of the flesh. What did Nicodemus understand? “Can a man enter a second time into his mother's womb, and be born?” Thus, whosoever shall tell you to be spiritually born a second time, answer in the words of Nicodemus, “Can a man enter a second time into his mother's womb, and be born?” I am already born of Adam, Adam cannot beget me a second time. I am already born of Christ, Christ cannot beget me again. As there is no repeating from the womb, so neither from baptism.
Source: Tractates on the Gospel of John (New Advent)