Tertullian, On Baptism 4

That much will do for a quick survey of the points where the rationale of baptism first comes into view—points that even then were already flagged, by the very nature of the thing, as a foreshadowing of baptism: the Spirit of God, who from the beginning hovered over the waters, was going to remake those who would be dipped (Genesis 1:2). The Holy One was certainly borne above the holy element; or else the element that bore him borrowed its holiness from the One borne above. Since any underlying matter inevitably takes on the quality of what looms over it—especially when the spiritual overtakes the bodily, easily penetrating and settling in through the sheer subtlety of its substance—the nature of the waters was sanctified by the Holy One, and the waters themselves began to sanctify. No one should ask: 'Are we actually baptized in those very 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. What belongs to the genus overflows into the species as well. Therefore it makes no difference whether someone is washed in sea or pool, river or spring, lake or trough, nor does it matter between those John baptized in the Jordan and those Peter baptized in the Tiber—unless that eunuch whom Philip baptized with happenstance water on the road received more or less salvation. So all waters, by virtue of the prerogative of their original origin, obtain the sacrament of sanctification once God is invoked. For the Spirit comes at once from heaven and rests upon the waters, sanctifying them from himself, and thus the sanctified waters imbibe the power to sanctify. Admittedly a simple analogy fits the simple act: just as we are polluted by sins in place of filth, we are washed with water. But sins do not appear on the flesh, because no one carries the stain of idolatry, or sexual immorality, or fraud on his skin; rather they defile the spirit, which is the author of the sin. The Spirit commands, the flesh serves; yet both share the guilt between them—the Spirit because it commands, the flesh because it carries out the order. Therefore, with the waters in a sense medicated through the Angel's intervention, the Spirit is diluted bodily in the waters, and the flesh is cleansed spiritually in the same.