Tertullian, On Baptism 4
That is enough on the points where the rationale for Baptism first comes into focus—beginning with the very pattern that, even then, foreshadowed the rite. The Spirit of God, who from the beginning hovered over the waters, was destined to remake those who would be dipped. The Holy One was borne over the holy element; or, if you prefer, the waters drew their sanctity from the One who hovered above them. Since any subject matter inevitably absorbs the quality of what hangs over it—especially the physical drawing from the spiritual, because spirit penetrates and indwells so easily by its subtlety—the nature of the waters was sanctified by the Holy One, and the waters themselves acquired the power to sanctify.
No one should ask: Are we baptized in those very same waters that existed at the start? Of course not—except insofar as the genus is one, even if the species are many. What belongs to the genus overflows into every species. So it makes no difference whether a person is washed in sea or pool, river or spring, lake or trough; nothing distinguishes those John baptized in the Jordan from those Peter baptized in the Tiber—unless the eunuch Philip baptized with happenstance water on the road somehow received more or less salvation. Therefore, all waters, by virtue of their original privilege, obtain the sacrament of sanctification once God is invoked. The Spirit immediately comes from heaven and rests upon the waters, sanctifying them from himself; thus sanctified, they drink in the power to sanctify.
True, the analogy fits the simple act: because sins pollute us like filth, we are washed with water. But sins do not appear on the flesh—no one carries the stain of idolatry, adultery, or fraud on the skin; they defile the spirit, which is the author of the sin. The spirit commands, the flesh serves; yet both share the guilt, the spirit for giving the order, the flesh for carrying it out. So when the waters are medicated, as it were, by the Angel's intervention, the spirit is washed bodily in the waters, and the flesh is cleansed spiritually in the same bath.
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.