5 The Lord says to Nicodemus, and explains to him: “Verily, verily, I say unto you, Unless a man be born again of water and of the Spirit, he cannot enter into the kingdom of God.” Thou, says He, understandest a carnal generation, when you say, Can a man return into his mother's bowels? The birth for the kingdom of God must be of water and of the Spirit. If one is born to the temporal inheritance of a human father, be he born of the bowels of a carnal mother; if one is born to the everlasting inheritance of God as his Father, be he born of the bowels of the Church. A father, as one that will die, begets a son by his wife to succeed him; but God begets of the Church sons, not to succeed Him, but to abide with Himself. And He goes on: “That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.” We are born spiritually then, and spirit we are born by the word and sacrament. The Spirit is present that we may be born; the Spirit is invisibly present whereof you are born, for you too must be invisibly born. For He goes on to say: “Marvel not that I said unto you, You must be born again. The Spirit blows where it lists, and you hear its voice, but know not whence it comes, or whither it goes.” None sees the Spirit; and how do we hear the Spirit's voice? There sounds a psalm, it is the Spirit's voice; the gospel sounds, it is the Spirit's voice; the divine word sounds, it is the Spirit's voice. “You hear its voice, and know not whence it comes, and whither it goes.” But if you are born of the Spirit, you too shall be so, that one who is not born of the Spirit knows not, as for you, whence you come, or whither you go. For He said, as He went on, “So is also every one that is born of the Spirit.”
6. “Nicodemus answered and said unto Him, How can these things be?” And, in fact, in the carnal sense, he knew not how. In him occurred what the Lord had said; the Spirit's voice he heard, but knew not whence it came, and whither it was going. “Jesus answered and said unto him, Are you a master in Israel, and know not these things?” Oh, brethren! What? Do we think that the Lord meant to taunt scornfully this master of the Jews? The Lord knew what He was doing; He wished the man to be born of the Spirit. No man is born of the Spirit if he be not humble, for humility itself makes us to be born of the Spirit; “for the Lord is near to them that are of broken heart.” The man was puffed up with his mastership, and it appeared of some importance to himself that he was a teacher of the Jews. Jesus pulled down his pride, that he might be born of the Spirit: He taunted him as an unlearned man; not that the Lord wished to appear his superior. What comparison can there be, God compared to man, truth to falsehood? Christ greater than Nicodemus! Ought this to be said, can it be said, is it to be thought? If it were said, “Christ is greater than angels,” it were ridiculous: for incomparably greater than every creature is He by whom every creature was made. But yet He rallies the man on his pride: “Are you a master in Israel, and know not these things?” As if He said, Behold, you know nothing, you are a proud chief; be born of the Spirit: for if you be born of the Spirit, you will keep the ways of God, so as to follow Christ's humility. So, indeed, is He high above all angels, that, “being in the form of God, He thought it not robbery to be equal with God, but emptied Himself, taking upon Him the form of a servant, being made into the likeness of men, and found in fashion as a man: He humbled Himself, being made obedient unto death” (and lest any kind of death should please you), “even the death of the cross.” He hung on the cross, and they scoffed at Him. He could have come down from the cross; but He deferred, that He might rise again from the tomb. He, the Lord, bore with proud slaves; the physician with the sick. If He did this, how ought they to act whom it behooves to be born of the Spirit!— if He did this, He who is the true Master in heaven, not of men only, but also of angels. For if the angels are learned, they are so by the Word of God. If they are learned by the Word of God, ask of what they are learned; and you shall find, “In the beginning was the Word, and the Word was with God, and the Word was God.” The neck of man is done away with, only the hard and stiff neck, that it may be gentle to bear the yoke of Christ, of which it is said, “My yoke is easy, and my burden is light.”
Source: Tractates on the Gospel of John (New Advent)