32 But you err, says my opponent, and are mistaken, and show, even in criticising these things, that you are rather ignorant, unlearned, and boorish. For all those stories which seem to you disgraceful, and tending to the discredit of the gods, contain in them holy mysteries, theories wonderful and profound, and not such as any one can easily become acquainted with by force of understanding. For that is not meant and said which has been written and placed on the surface of the story; but all these things are understood in allegorical senses, and by means of secret explanations privately supplied. Therefore he who says Jupiter lay with his mother, does not mean the incestuous or shameful embraces of Venus, but names Jupiter instead of rain, and Ceres instead of the earth.
And he, again, who says that he dealt lasciviously with his daughter, speaks of no filthy pleasures, but puts Jupiter for the name of a shower, and by his daughter means the crop sown. So, too, he who says that Proserpina was carried off by father Dis, does not say, as you suppose, that the maiden was carried off to gratify the basest desires; but because we cover the seed with clods, he signifies that the goddess has sunk under the earth, and unites with Orcus to bring forth fruit. In like manner in the other stories also one thing indeed is said, but something else is understood; and under a commonplace openness of expression there lurks a secret doctrine, and a dark profundity of mystery.
Source: Against the Heathen (New Advent)