Moreover, if the law of Moses had contained nothing which was to be understood as having a secret meaning, the prophet would not have said in his prayer to God, “Open my eyes, and I will behold wondrous things out of Your law;” whereas he knew that there was a veil of ignorance lying upon the heart of those who read but do not understand the figurative meaning, which veil is taken away by the gift of God, when He hears him who has done all that he can, and who by reason of habit has his senses exercised to distinguish between good and evil, and who continually utters the prayer, “Open my eyes, and I will behold wondrous things out of Your law.”
And who is there that, on reading of the dragon that lives in the Egyptian river, and of the fishes which lurk in his scales, or of the excrement of Pharaoh which fills the mountains of Egypt, is not led at once to inquire who he is that fills the Egyptian mountains with his stinking excrement, and what the Egyptian mountains are; and what the rivers in Egypt are, of which the aforesaid Pharaoh boastfully says, “The rivers are mine, and I have made them;” and who the dragon is, and the fishes in its scales—and this so as to harmonize with the interpretation to be given of the rivers?
But why establish at greater length what needs no demonstration? For to these things applies the saying: “Who is wise, and he shall understand these things? Or who is prudent, and he shall know them?” Now I have gone at some length into the subject, because I wished to show the unsoundness of the assertion of Celsus, that “the more modest among the Jews and Christians endeavour somehow to give these stories an allegorical signification, although some of them do not admit of this, but on the contrary are exceedingly silly inventions.”
Much rather are the stories of the Greeks not only very silly, but very impious inventions. For our narratives keep expressly in view the multitude of simpler believers, which was not done by those who invented the Grecian fables. And therefore not without propriety does Plato expel from his state all fables and poems of such a nature as those of which we have been speaking.
Source: Against Celsus (New Advent)