Tertullian, On Baptism 4
This much will do for the points where the logic of baptism first comes into focus. Already back then the very act foreshadowed the rite: the Spirit of God, hovering over the waters at creation (Genesis 1:2), was destined to remake those who would be dipped. The Holy One was borne over the holy element; either the water received holiness from the Spirit above it, or the Spirit imparted holiness to the water he rode. Since any substance underneath inevitably absorbs the quality of what hovers above — especially when the spiritual penetrates the bodily, slipping in and settling deep by sheer fineness of substance — the waters were sanctified by the Holy One and themselves gained the power to sanctify. No one should ask whether we are baptized in the exact waters that existed at the beginning. Of course not — except that the genus is one, even if the species are many. Whatever belongs to the genus overflows into every species. So it makes no difference whether you are washed in sea or pool, river or spring, lake or trough; there is no distinction between those John baptized in the Jordan and those Peter baptized in the Tiber. Unless you think the eunuch Philip baptized with roadside water on the way got more or less salvation. Therefore, by the prerogative of their original origin, all waters obtain the sacrament of sanctification once God is invoked. The Spirit descends at once from heaven, rests on the waters, sanctifies them from himself, and the sanctified waters then drink in the power to sanctify. Admittedly, the simple analogy fits the act: just as we are stained by sins instead of dirt, we are washed with water. But sins do not show on the flesh — no one wears the stain of idolatry, adultery, or fraud on his skin. They defile the spirit, the author of the offense. The spirit commands; the flesh obeys. Yet both share the guilt: the spirit for giving the order, the flesh for carrying it out. Accordingly, once the waters are medicated, so to speak, by the Angel's intervention, the spirit is washed bodily in the water and the flesh is cleansed spiritually in the same water.
Latin
Sed ea satis erit praecerpsisse, in quibus et ratio Baptismi recognoscitur prima illa, qua jam tunc etiam ipso habitu praenotabatur ad Baptismi figuram, Dei spiritum, qui ab initio supervectabatur super aquas, intinctos reformaturum. Sanctum autem utique super sanctum ferebatur; aut ab eo quod superferebatur, id quod ferebat, sanctitatem mutuabatur. Quoniam subjecta quaeque materia, ejus quae desuper imminet, qualitatem rapiat necesse est, maxime corporalis spiritalem, et penetrare et insidere facilem per substantiae suae subtilitatem. Ita de Sancto sanctificata natura aquarum, et ipsa sanctificare concepit. Nemo dicat: Numquid ipsis enim aquis tinguimur quae tunc in primordio fuerunt? non utique ipsis nisi ex ea parte, qua genus quidem unum, species vero complures. Quod autem generi attributum est, etiam in specie redundat. Ideoque nulla distinctio est, mari quis an stagno, flumine an fonte, lacu an alveo diluatur, nec quidquam refert inter eos quos Joannes in Jordane, et quos Petrus in Tiberi tinxit. Nisi et ille spado, quem Philippus inter vias fortuita aqua tinxit, plus salutis aut minus retulit. Igitur omnes aquae de pristina originis praerogativa sacramentum sanctificationis consequuntur, invocato Deo. Supervenit enim statim spiritus de coelis, et aquis superest, sanctificans eas de semetipso, et ita sanctificatae vim sanctificandi combibunt. Quanquam ad simplicem actum competat similitudo, ut, quoniam vice sordium delictis inquinamur, aquis abluamur. Sed delicta, sicut non in carne comparent, quia nemo super cutem portat maculam idololatriae, aut stupri, aut fraudis; ita ejusmodi in spiritu sordent, qui est auctor delicti. Spiritus enim dominatur, caro famulatur; tamen utromque inter se communicant reatum, spiritus ob imperium, caro ob ministerium. Igitur medicatis quodammodo aquis per Angeli interventum, et spiritus in aquis corporaliter diluitur, et caro in eisdem spiritaliter mundatur.