From the treatise On the Mysteries by Saint Ambrose, bishop
We are born again of water and the Holy Spirit
What did you see in the baptistery? Water certainly, but not water alone. You saw the levites ministering there, the high priest asking questions and consecrating. First of all, the Apostle taught you that we must fix our eyes, not on the things that are seen but on the things that are unseen, for the things that are seen are for a time, but the things that are unseen are eternal. In another place you may read that the invisible things of God, from the creation of the world, can be understood through the things that have been created, and his everlasting power and godhead can be known through his works. The Lord himself says: If you do not believe me, believe at least my works. Then believe that the presence of the godhead is there. You believe in its activity, and refuse to believe in its presence? How could there be activity if there were no presence beforehand?
Consider how ancient the mystery is, prefigured as it was in the creation of the world itself. In the very beginning, when God made heaven and earth, the spirit, God tells us, moved over the waters. Was the spirit not active as he moved over the waters? When the prophet tells you that by the word of the Lord the heavens were established, and by the spirit of his mouth all their array, realize that the spirit was active in this making of the world. The fact that he moved over the waters, and the fact that he was active, both rest on prophetic testimony. Moses tells us that the spirit moved over the waters; David testifies that the spirit was active.
Listen to another testimony. All flesh had become corrupt because of its sins. God said: My spirit will not remain in men, for they are flesh. God thus shows that spiritual grace is repelled by uncleanness of the flesh and by the stain of more serious sin. So God resolved to restore the gift he had given. He sent the flood and ordered Noah, the righteous man, into the ark. When the flood began to subside Noah sent first a raven, then a dove, which as we read, came back with an olive branch. You see water, you see wood, you look on a dove, and you hesitate to believe the mystery?
The water is that in which the flesh is dipped, to wash away all its sin. In it all wickedness is buried. The wood is that to which the Lord Jesus was fastened when he suffered for us. The dove is the one in whose likeness the Holy Spirit descended, as you have learned from the New Testament: the Spirit who breathes into you peace of soul, tranquillity of mind.