Tertullian, On Baptism 4

I've said enough on the points where the logic of Baptism first surfaces—points that already foreshadowed, by the mere fact of the Spirit's presence, how the Spirit of God, who hovered over the waters at creation, would one day reform those who are dipped. The Holy Spirit was certainly borne over what is holy, and it borrowed holiness from the holy element it hovered above. Because any underlying matter inevitably takes on the quality of what hangs over it—especially when the spiritual overshadows the physical—and because the spiritual easily penetrates and dwells in the physical through the subtlety of its substance, the nature of the waters was sanctified by the Holy One and thereby gained the power to sanctify in turn. No one should ask: Are we actually baptized in the very same waters that existed at the beginning? Of course not in those identical waters, except insofar as the genus is one while the species are many; whatever belongs to the genus also overflows into every species. Consequently, it makes no difference whether someone is washed in the sea or a pool, a river or a spring, a lake or a basin, nor does it matter between those John baptized in the Jordan and those Peter baptized in the Tiber—unless you think the eunuch Philip baptized with chance water on the road received more or less salvation. Therefore, all waters, by the prerogative of their original source, obtain the sacrament of sanctification once God is invoked. For the Spirit immediately comes from heaven and remains upon the waters, sanctifying them from itself, and these sanctified waters then imbibe the very power to sanctify. Admittedly, the analogy fits a simple act: just as we are defiled by sins, we are washed with water. But sins do not show up on the flesh, because no one carries the stain of idolatry, adultery, or fraud on their skin; rather, such things defile the spirit, which is the author of sin. The spirit dominates, the flesh serves; yet both share the guilt—the spirit because it commands, the flesh because it executes. So, with the waters somehow healed through the Angel's intervention, the spirit is corporally washed in the waters while the flesh is spiritually cleansed in them.