St. Ambrose points out that we must consider the divine presence and working in the water and the sacred ministers, and then brings forward many Old Testament figures of baptism.
8. What did you see? Water, certainly, but not water alone; you saw the deacons ministering there, and the bishop asking questions and hallowing. First of all, the Apostle taught you that those things are not to be considered “which we see, but the things which are not seen, for the things which are seen are temporal, but the things which are not seen are eternal.” For you read elsewhere: “That the invisible things of God, since the creation of the world, are understood through those things which have been made; His eternal power also and Godhead are estimated by His works.” Wherefore also the Lord Himself says: “If you believe not Me, believe at least the works.” Believe, then, that the presence of the Godhead is there. Do you believe the working, and not believe the presence? Whence should the working proceed unless the presence went before?
9. Consider, however, how ancient is the mystery prefigured even in the origin of the world itself. In the very beginning, when God made the heaven and the earth, “the Spirit,” it is said, “moved upon the waters.” He Who was moving upon the waters, was He not working upon the waters? But why should I say, “working”? As regards His presence He was moving. Was He not working Who was moving? Recognize that He was working in that making of the world, when the prophet says: “By the word of the Lord were the heavens made, and all their strength by the spirit of His mouth.” Each statement rests upon the testimony of the prophet, both that He was moving and that He was working. Moses says that He was moving, David testifies that he was working.
10. Take another testimony. All flesh was corrupt by its iniquities. “My Spirit,” says God, “shall not remain among men, because they are flesh.” Whereby God shows that the grace of the Spirit is turned away by carnal impurity and the pollution of grave sin. Upon which, God, willing to restore what was lacking, sent the flood and bade just Noah go up into the ark. And he, after having, as the flood was passing off, sent forth first a raven which did not return, sent forth a dove which is said to have returned with an olive twig. You see the water, you see the wood [of the ark], you see the dove, and do you hesitate as to the mystery?
11. The water, then, is that in which the flesh is dipped, that all carnal sin may be washed away. All wickedness is there buried. The wood is that on which the Lord Jesus was fastened when He suffered for us. The dove is that in the form of which the Holy Spirit descended, as you have read in the New Testament, Who inspires in you peace of soul and tranquillity of mind. The raven is the figure of sin, which goes forth and does not return, if, in you, too, inwardly and outwardly righteousness be preserved.
12. There is also a third testimony, as the Apostle teaches us: “For all our fathers were under the cloud, and all passed through the sea, and were all baptized to Moses in the cloud and in the sea.” And further, Moses himself says in his song: “You sent Your Spirit, and the sea covered them.” You observe that even then holy baptism was prefigured in that passage of the Hebrews, wherein the Egyptian perished, the Hebrew escaped. For what else are we daily taught in this sacrament but that guilt is swallowed up and error done away, but that virtue and innocence remain unharmed?
13. You hear that our fathers were under the cloud, and that a kindly cloud, which cooled the heat of carnal passions. That kindly cloud overshadows those whom the Holy Spirit visits. At last it came upon the Virgin Mary, and the Power of the Highest overshadowed her, when she conceived Redemption for the race of men. And that miracle was wrought in a figure through Moses. If, then, the Spirit was in the figure, is He not present in the reality, since Scripture says to us: “For the law was given by Moses, but grace and truth came by Jesus Christ.”
14. Marah was a fountain of most bitter water: Moses cast wood into it and it became sweet. For water without the preaching of the Cross of the Lord is of no avail for future salvation, but, after it has been consecrated by the mystery of the saving cross, it is made suitable for the use of the spiritual laver and of the cup of salvation. As, then, Moses, that is, the prophet, cast wood into that fountain, so, too, the priest utters over this font the proclamation of the Lord's cross, and the water is made sweet for the purpose of grace.
15. You must not trust, then, wholly to your bodily eyes; that which is not seen is more really seen, for the object of sight is temporal, but that other eternal, which is not apprehended by the eye, but is discerned by the mind and spirit.
16. Lastly, let the lessons lately gone through from the Kings teach you. Naaman was a Syrian, and suffered from leprosy, nor could he be cleansed by any. Then a maiden from among the captives said that there was a prophet in Israel, who could cleanse him from the defilement of the leprosy. And it is said that, having taken silver and gold, he went to the king of Israel. And he, when he heard the cause of his coming, rent his clothes, saying, that occasion was rather being sought against him, since things were asked of him which pertained not to the power of kings. Elisha, however, sent word to the king, that he should send the Syrian to him, that he might know there was a God in Israel. And when he had come, he bade him dip himself seven times in the river Jordan.
17. Then he began to reason with himself that he had better waters in his own country, in which he had often bathed and never been cleansed of his leprosy; and so remembering this, he did not obey the command of the prophet, yet on the advice and persuasion of his servants he yielded and dipped himself. And being immediately cleansed, he understood that it is not of the waters but of grace that a man is cleansed.
18. Understand now who is that young maid among the captives. She is the congregation gathered out of the Gentiles, that is, the Church of God held down of old by the captivity of sin, when as yet it possessed not the liberty of grace, by whose counsel that foolish people of the Gentiles heard the word of prophecy as to which it had before been in doubt. Afterwards, however, when they believed that it ought to be obeyed, they were washed from every defilement of sin. And he indeed doubted before he was healed; you are already healed, and therefore ought not to doubt.
Source: On the Mysteries (New Advent)