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.”
Source: Tractates on the Gospel of John (New Advent)