36 But you will perhaps say that these allegories are not found in the whole body of the story, but that some parts are written so as to be understood by all, while others have a double meaning, and are veiled in ambiguity. That is refined subtlety, and can be seen through by the dullest. For because it is very difficult for you to transpose, reverse, and divert to other meanings all that has been said, you choose out some things which suit your purpose, and by means of these you strive to maintain that false and spurious versions were thrown about the truth which is under them. But yet, supposing that we should grant to you that it is just as you say, how do you know, or whence do you learn, which part of the story is written without any double meaning, which, on the other hand, has been covered with jarring and alien senses?
For it may be that what you believe to be so is otherwise, that what you believe to be otherwise has been produced with different, and even opposite modes of expression. For where, in a consistent whole, one part is said to be written allegorically, the other in plain and trustworthy language, while there is no sign in the thing itself to point out the difference between what is said ambiguously and what is said simply, that which is simple may as well be thought to have a double meaning, as what has been written ambiguously be believed to be wrapt in obscurity. But, indeed, we confess that we do not understand at all by whom this is either done, or can be believed to be possible.
Source: Against the Heathen (New Advent)