From the treatise "On the Mysteries" by St Ambrose, bishop

 

We are born again  through water and the Holy Spirit

What did you see at the baptism? Water, certainly, but not water alone; you saw the deacons (like the Levites of old) exercising their ministry and the bishop (like the chief priest of old) asking questions and bestowing sanctification.

  The Apostle Paul taught you to look not at what is visible but at what is invisible; for visible things will pass away but the invisible things are eternal. As you read elsewhere: Since the creation of the world, the invisible attributes of God, his eternal power and his divinity are understood through the things that he has done. The Lord himself says: If you do not believe in me, believe in my works. So here, at baptism, believe that the Godhead is present. Can you believe that God is at work and yet deny that he is present? How can any work happen unless the one who performs it is already there?

  Consider how ancient this mystery is; for it is prefigured even in the origin of the world itself. In the very beginning, when God made the heaven and the earth, it is said: The Spirit moved upon the waters. He who was moving over the waters, was he not acting on them as well? You can recognize that he was working in that moment of creation, when you see how the prophet says: By the word of the Lord were the heavens made, and all their strength by the spirit of his mouth. There is as much support from the prophets for one thing as for the other. Moses says that the spirit of God was moving and David the psalmist testifies that he was working.

  Here is another piece of evidence. By its own iniquities all flesh was corrupted. And God says: My Spirit shall not remain among men, because they are flesh. This goes to show that carnal impurity and the pollution of grave sin turn away the grace of the Spirit. Since that had happened, God sought to repair his disfigured creation. He sent the flood and commanded Noah, the just man, to go up into the ark. As the waters of the flood were receding Noah sent first a raven (which did not return) and then a dove, which came back with an olive branch, as we read in the scriptures. And now you see the water, you see the wood, you see the dove, and you still doubt the mystery?

  The water is the water into which the flesh is dipped, to wash away all the sins of the flesh. And so is all sin buried.

  The wood is the wood on which the Lord Jesus was fastened when he suffered for us.

  The dove symbolizes the Holy Spirit’s taking on the form of a dove, as you have learnt from the New Testament: the Sprit who brings peace to your soul and calm to your troubled mind.