46 But, says my opponent, if he was not a god, why, after he left the ship, and crawled to the island in the Tiber, did he immediately become invisible, and cease to be seen as before? Can we indeed know whether there was anything in the way under cover of which he hid himself, or any opening in the earth? Do you declare, say yourselves, what that was, or to what race of beings it should be referred, if your service of certain personages is in itself certain. Since the case is thus, and the discussion deals with your deity, and your religion also, it is your part to teach, and yours to show what that was, rather than to wish to hear our opinions and to await our decisions.
For we, indeed, what else can we say than that which took place and was seen, which has been handed down in all the narratives, and has been observed by means of the eyes? This, however, undoubtedly we say was a colubra of very powerful frame and immense length, or, if the name is despicable, we say it was a snake, we call it a serpent, or any other name which usage has afforded to us, or the development of language devised. For if it crawled as a serpent, not supporting itself and walking on feet, but resting upon its belly and breast; if, being made of fleshly substance, it lay stretched out in slippery length; if it had a head and tail, a back covered with scales, diversified by spots of various colours; if it had a mouth bristling with fangs, and ready to bite, what else can we say than that it was of earthly origin, although of immense and excessive size, although it exceeded in length of body and greatness of might that which was slain by Regulus by the assault of his army?
But if we think otherwise, we subvert and overthrow the truth. It is yours, then, to explain what that was, or what was its origin, its name, and nature. For how could it have been a god, seeing that it had those things which we have mentioned, which gods should not have if they intend to be gods, and to possess this exalted title? After it crawled to the island in the Tiber, immediately it was nowhere to be seen, by which it is shown that it was a deity. Can we, then, know whether there was there anything in the way under cover of which it hid itself, or some opening in the earth, or some caverns and vaults, caused by huge masses being heaped up irregularly, into which it hurried, evading the gaze of the beholders?
For what if it leaped across the river? What if it swam across it? What if it hid itself in the dense forests? It is weak reasoning from this, to suppose that that serpent was a god because with all speed it withdrew itself from the eyes of the beholders, since, by the same reasoning, it can be proved, on the other hand, that it was not a god.
Source: Against the Heathen (New Advent)