In one of the best parts, St. Ambrose looks at how the Bible, from Genesis forwards, testifies to regenerative Baptism - that God will save us through water. His examples from Genesis are Noah's Ark (which St. Peter uses for the same point in 1 Peter 3:19-22), and Genesis 1:2, an allusion to Baptism most of us overlook:
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." [Genesis 1:2] 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." [Psalm 33:6] 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." [Genesis 6:3] 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.Noah and the Ark is described in Genesis 7 and 8, but the key passage for Ambrose seems to be Genesis 8:6-12. St. Augustine, commenting on the same passage, had something similar to say on it:
That the raven sent out after forty days did not return, being either prevented by the water or attracted by some floating carcass; as men defiled by impure desire, and therefore eager for things outside in the world, are either baptized, or are led astray into the company of those to whom, as they are outside the ark, that is, outside the Church, baptism is destructive.
That the dove when sent forth found no rest, and returned; as in the New Testament rest is not promised to the saints in this world. The dove was sent forth after forty days, a period denoting the length of human life.
When again sent forth after seven days, denoting the sevenfold operation of the Spirit, the dove brought back a fruitful olive branch; as some even who are baptized outside of the Church, if not destitute of the fatness of charity, may come after all, as it were in the evening, and be brought into the one communion by the mouth of the dove in the kiss of peace.
That, when again sent forth after seven days, the dove did not return; as, at the end of the world, the rest of the saints shall no longer be in the sacrament of hope, as now, while in the communion of the Church, they drink what flowed from the side of Christ, but in the perfection of eternal safety, when the kingdom shall be delivered up to God and the Father, and when, in that unclouded contemplation of unchangeable truth, we shall no longer need natural symbols.